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[edit] Introduction

C# (pronounced "See Sharp") is a multi-purpose computer programming language suitable for all development needs.

[edit] Introduction

Although C# is derived from the C programming language, it has features such as garbage collection that allow beginners to become proficient in C# more quickly than in C or C++. Similar to Java, it is object-oriented, comes with an extensive class library, and supports exception handling, multiple types of polymorphism, and separation of interfaces from implementations. Those features, combined with its powerful development tools, multi-platform support, and generics, make C# a good choice for many types of software development projects: rapid application development projects, projects implemented by individuals or large or small teams, Internet applications, and projects with strict reliability requirements. Testing frameworks such as NUnit make C# amenable to test-driven development and thus a good language for use with Extreme Programming (XP). Its strong typing helps to prevent many programming errors that are common in weakly typed languages.

A large part of the power of C# (as with other .NET languages), comes with the common .NET Framework API, which provides a large set of classes, including ones for encryption, TCP/IP socket programming, and graphics. Developers can thus write part of an application in C# and another part in another .NET language (e.g. VB .NET), keeping the tools, library, and object-oriented development model while only having to learn the new language syntax.

Because of the similarities between C# and the C family of languages, as well as Java, a developer with a background in object-oriented languages like C++ may find C# structure and syntax intuitive.

[edit] Standard

Wikipedia has related information at C Sharp (programming language)

Microsoft, with Anders Hejlsberg as Chief Engineer, created C# as part of their .NET initiative and subsequently opened its specification via the ECMA. Thus, the language is open to implementation by other parties. Other implementations include Mono and DotGNU.

C# and other .NET languages rely on an implementation of the virtual machine specified in the Common Language Infrastructure, like Microsoft's Common Language Runtime (CLR). That virtual machine manages memory, handles object references, and performs Just-In-Time (JIT) compiling of Common Intermediate Language code. The virtual machine makes C# programs safer than those that must manage their own memory and is one of the reasons .NET language code is referred to as managed code. More like Java than C and C++, C# discourages explicit use of pointers, which could otherwise allow software bugs to corrupt system memory and force the operating system to halt the program forcibly with nondescript error messages.

[edit] History

Microsoft's original plan was to create a rival to Java, named J++ but this was abandoned to create C#, codenamed "Cool".

Microsoft submitted C# to the ECMA standards group mid-2000.

C# 2.0 was released in late-2005 as part of Microsoft's development suite, Visual Studio 2005. The 2.0 version of C# includes such new features as generics, partial classes, and iterators.

[1][2]

To compile your first C# application, you will need a copy of a .NET Framework SDK installed on your PC.

There are two .NET frameworks available: Microsoft's and Mono's.

Microsoft

For Windows, the .NET Framework SDK can be downloaded from Microsoft's .NET Framework Developer Center. If the default Windows directory (the directory where Windows or WinNT is installed) is C:\WINDOWS, the .Net Framework SDK installation places the Visual C# .NET Compiler (csc) in the C:\WINDOWS\Microsoft.NET\Framework\v1.0.3705 directory for version 1.0, the C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322 directory for version 1.1, or the C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727 directory for version 2.0.

Mono

For Windows, Linux, or other Operating Systems, an installer can be downloaded from the Mono website.

For Linux, a good compiler is cscc which can be downloaded for free from the DotGNU Portable.Net project page. The compiled programs can then be run with ilrun.

If you are working on Windows it is a good idea to add the path to the folders that contain cs.exe or mcs.exe to the Path environment variable so that you do not need to type the full path each time you want to compile.

For writing C#.NET code, there are plenty of editors that are available. It's entirely possible to write C#.NET programs with a simple text editor, but it should be noted that this requires you to compile the code yourself. Microsoft offers a wide range of code editing programs under the Visual Studio line that offer syntax highlighting as well as compiling and debugging capabilities. Currently C#.NET can be compiled in Visual Studio 2002 and 2003 (only supports the .NET Framework version 1.0 and 1.1) and Visual Studio 2005 (supports the .NET Framework 2.0 and earlier versions with some tweaking). Microsoft offers five Visual Studio editions, four of which are sold commercially. The Visual Studio C# Express Edition can be downloaded and used for free from Microsoft's website.

The code below will demonstrate a C# program written in a simple text editor. Start by saving the following code to a text file called hello.cs:

using System;

namespace MyConsoleApplication
{
	class MyFirstClass
	{
		static void Main(string[] args)
		{
			System.Console.WriteLine("Hello,");
			Console.WriteLine("World!");

			Console.ReadLine();
		}
	}
}

To compile hello.cs, run the following from the command line:

• For standard Microsoft installations of .NET 2.0, run C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\csc.exe hello.cs
• For Mono run mcs hello.cs.
• For users of cscc, compile with cscc hello.cs -o hello.exe.

Doing so will produce hello.exe. The following command will run hello.exe:

• On Windows, use hello.exe.
• On Linux, use mono hello.exe or ilrun hello.exe.

Alternatively, in Visual C# express, you could just hit F5 or the green play button to run the code. If you want to run without debugging, press CTRL-F5.

Running hello.exe will produce the following output:

Hello,
World!

The program will then wait for you to strike 'enter' before returning to the command prompt.

Note that the example above includes the System namespace via the using keyword. That inclusion allows direct references to any member of the System namespace without specifying its fully qualified name.

The first call to the WriteLine method of the Console class uses a fully qualified reference.

System.Console.WriteLine("Hello,");

The second call to that method shortens the reference to the Console class by taking advantage of the fact that the System namespace is included (with using System).

Console.WriteLine("World!");

C# is a fully object-oriented language. The following sections explain the syntax of the C# language as a beginner's course for programming in the language. Note that much of the power of the language comes from the classes provided with the .NET framework, which are not part of the C# language syntax per se.

[edit] Language Basics

This section will define the naming conventions that are generally accepted by the C# development community. Some companies may define naming conventions that differ from this, but that is done on an individual basis and is generally discouraged. Some of the objects discussed in this section may be beyond the reader's knowledge at this point, but this section can be referred back to later.

[edit] Reasoning

Much of the naming standards are derived from Microsoft's .NET Framework libraries. These standards have proven to make names readable and understandable "at a glance". By using the correct conventions when naming objects, you ensure that other C# programmers who read your code will easily understand what objects are without having to search your code for their definition.

[edit] Conventions

[edit] Namespace

Namespaces are named using Pascal Case (also called UpperCamelCase) with no underscores. This means the first letter of every word in the name is capitalized. For example: MyNewNamespace. Also, note that Pascal Case also denotes that acronyms of three or more letters should only have the first letter capitalized (MyXmlNamespace instead of MyXMLNamespace)

[edit] Assemblies

If an assembly contains only one namespace, they should use the same name. Otherwise, Assembles should follow the normal Pascal Case format.

[edit] Classes and Structures

Pascal Case, no underscores or leading "C", "cls", or "I". Classes should not have the same name as the namespace in which they reside. Any acronyms of three or more letters should be pascal case, not all caps. Try to avoid abbreviations, and try to always use nouns.

[edit] Exception Classes

Follow class naming conventions, but add Exception to the end of the name. In .Net 2.0, all classes should inherit from the System.Exception base class, and not inherit from the System.ApplicationException.

[edit] Interfaces

Follow class naming conventions, but start the name with "I" and capitalize the letter following the "I". Example: IFoo The "I" prefix helps to differentiate between Interfaces and classes and also to avoid name collisions.

[edit] Functions

Pascal Case, no underscores except in the event handlers. Try to avoid abbreviations. Many programmers have a nasty habit of overly abbreviating everything. This should be discouraged.

[edit] Properties and Public Member Variables

Pascal Case, no underscores. Try to avoid abbreviations.

[edit] Parameters and Procedure-level Variables

Camel Case (or lowerCamelCase). Try to avoid abbreviations. Camel Case is the same as Pascal case, but the first letter of the first word is lowercased.

[edit] Class-level Private and Protected Variables

Camel Case with a leading underscore. Always indicate "Protected" or "Private" in the declaration. The leading underscore is the only controversial thing in this document. The leading character helps to prevent name collisions in constructors (a parameter and a private variable have the same name).

[edit] Controls on Forms

Pascal Case with a prefix that identifies it as being part of the UI instead of a purely coded control (example a temporary variable). Many developers use "ui" as the prefix followed by a descriptive name such as "UserNameTextBox"

[edit] Constants

Pascal Case. The use of SCREAMING_CAPS is discouraged. This is a large change from earlier conventions. Most developers now realize that in using SCREAMING_CAPS they betray more implementation than is necessary. A large portion of the .NET Framework Design Guidelines is dedicated to this discussion.

[edit] Example

Here is an example of a class that uses all of these naming conventions combined.

using System;
 
namespace MyExampleNamespace
{
    public class Customer : IDisposable
    {
        private string _customerName;
        public string CustomerName 
        { 
            get 
            { 
                return _customerName; 
            }
            set
            {
                _customerName = value;
                _lastUpdated = DateTime.Now;
            }
        }
 
        private DateTime _lastUpdated;
 
        public DateTime LastUpdated
        {
            get
            {
                return _lastUpdated;
            }
            private set
            {
                _lastUpdated = value;
            }
        }
 
        public void UpdateCustomer(string newName)
        {
            if( !newName.Equals(CustomerName))
            {
                CustomerName = newName;
            }
        }
 
        public void Dispose()
        {
            //Do nothing
        }
    }
}

C# syntax looks quite similar to the syntax of Java because both inherit much of their syntax from C and C++. The object-oriented nature of C# requires the high-level structure of a C# program to be defined in terms of classes, whose detailed behaviors are defined by their statements.

[edit] Statements

The basic unit of execution in a C# program is the statement. A statement can declare a variable, define an expression, perform a simple action by calling a method, control the flow of execution of other statements, create an object, or assign a value to a variable, property, or field. Statements are usually terminated by a semicolon.

Statements can be grouped into comma-separated statement lists or brace-enclosed statement blocks.

Examples:

int sampleVariable;                           // declaring a variable
sampleVariable = 5;                           // assigning a value
Method();                                     // calling an instance method
SampleClass sampleObject = new SampleClass(); // creating a new instance of an object
sampleObject.ObjectMethod();                  // calling a member function of an object
 
// executing a "for" loop with an embedded "if" statement 
for(int i = 0; i < upperLimit; i++)
{
    if (SampleClass.SampleStaticMethodReturningBoolean(i))
    {
        sum += sampleObject.SampleMethodReturningInteger(i);
    }
}

[edit] Statement blocks

A series of statements surrounded by curly braces form a block of code. Among other purposes, code blocks serve to limit the scope of variables defined within them. Code blocks can be nested and often appear as the bodies of methods.

private void MyMethod(int value)
{  // This block of code is the body of "MyMethod()"
 
   // The 'value' integer parameter is accessible to everything in the method
 
   int methodLevelVariable; // This variable is accessible to everything in the method
 
   if (value == 2)
   {
      // methodLevelVariable is still accessible here     
 
      int limitedVariable; // This variable is only accessible to code in the if block
 
      DoSomeWork(limitedVariable);
   }
 
   // limitedVariable is no longer accessible here
 
}  // Here ends the code block for the body of "MyMethod()".

[edit] Comments

Comments allow inline documentation of source code. The C# compiler ignores comments. These styles of comments are allowed in C#:

Single-line comments

The "//" character sequence marks the following text as a single-line comment. Single-line comments, as one would expect, end at the first end-of-line following the "//" comment marker.

Multiple-line comments

Comments can span multiple lines by using the multiple-line comment style. Such comments start with "/*" and end with "*/". The text between those multi-line comment markers is the comment.

//This style of a comment is restricted to one line.
/* 
   This is another style of a comment.
   It allows multiple lines.
*/

XML Documentation-line comments

These comments are used to generate XML documentation. Single-line and multiple-line styles can be used. The single-line style, where each line of the comment begins with "///", is more common than the multiple-line style delimited by "/**" and "*/".

/// <summary> documentation here </summary>
/// <remarks>
///     This uses single-line style XML Documentation comments.
/// </remarks>
 
 
/** 
 * <summary> documentation here </summary>
 * <remarks>
 *     This uses multiple-line style XML Documentation comments.
 * </remarks>
 */

[edit] Case sensitivity

C# is case-sensitive, including its variable and method names.

The variables myInteger and MyInteger below are distinct because C# is case-sensitive:

int myInteger = 3;
int MyInteger = 5;

For example, C# defines a class Console to handle most operations with the console window. Writing the following code would result in a compiler error unless an object named console had been previously defined.

 // Compiler error!
 console.writeline("Hello");

The following corrected code compiles as expected because it uses the correct case:

 Console.WriteLine("Hello");

Variables are used to store values. More technically, a variable binds an object (in the general sense of the term, i.e. a specific value) to an identifier (the variable's name) so that the object can be accessed later. Variables can, for example, store a value for later use:

string name = "Dr. Jones";
Console.WriteLine("Good morning " + name);

In this example "name" is the identifier and "Dr. Jones" is the value that we bound to it. Also, each variable is declared with an explicit type. Only values whose types are compatible with the variable's declared type can be bound to (stored in) the variable. In the above example we stored "Dr. Jones" into a variable of the type string. This is a legal statement. However, if we had said int name = "Dr. Jones", the compiler would have thrown an error telling us that you cannot implicitly convert between int and string. There are methods for doing this, but we will talk about them later.

[edit] Fields, Local Variables, and Parameters

C# supports several program elements corresponding to the general programming concept of variable: fields, parameters, and local variables.

[edit] Fields

Fields, sometimes called class-level variables, are variables associated with classes or structures. An instance variable is a field associated with an instance of the class or structure, while a static variable, declared with the static keyword, is a field associated with the type itself. Fields can also be associated with their class by making them constants (const), which requires a declaration assignment of a constant value and prevents subsequent changes to the field.

Each field has a visibility of public, protected, internal, protected internal, or private (from most visible to least visible).

[edit] Local variables

Like fields, local variables can optionally be constant (const). Constant local variables are stored in the assembly data region, while non-constant local variables are stored on (or referenced from) the stack. They thus have both a scope and an extent of the method or statement block that declares them.

[edit] Parameter

Parameters are variables associated with a method.

An in parameter may either have its value passed in from the callee to the method's environment, so that changes to the parameter by the method do not affect the value of the callee's variable, or passed in by reference, so that changes to the variables will affect the value of the callee's variable. Value types (int, double, string) are passed in "by value" while reference types (objects) are passed in "by reference." Since this is the default for the C# compiler, it is not necessary to use .

An out parameter does not have its value copied, thus changes to the variable's value within the method's environment directly affect the value from the callee's environment. Such a variable is considered by the compiler to be unbound upon method entry, thus it is illegal to reference an out parameter before assigning it a value. It also must be assigned by the method in each valid (non-exceptional) code path through the method in order for the method to compile.

A reference parameter is similar to an out parameter, except that it is bound before the method call and it need not be assigned by the method.

A params parameter represents a variable number of parameters. If a method signature includes one, the params argument must be the last argument in the signature.

// Each pair of lines is what the definition of a method and a call of a 
//   method with each of the parameters types would look like.
// In param:
void MethodOne(int param1) //definition
MethodOne(variable);          //call
 
// Out param:
void MethodTwo(out string message)  //definition
MethodTwo(out variable);            //call
 
// Reference param;
void MethodThree(ref int someFlag)  //definition
MethodThree(ref theFlag)            //call
 
// Params
void MethodFour(params string[] names)           //definition
MethodFour("Matthew", "Mark", "Luke", "John"); //call

[edit] Types

Each type in C# is either a value type or a reference type. C# has several predefined ("built-in") types and allows for declaration of custom value types and reference types.

[edit] Integral types

Because the type system in C# is unified with other languages that are CLI-compliant, each integral C# type is actually an alias for a corresponding type in the .NET framework. Although the names of the aliases vary between .NET languages, the underlying types in the .NET framework remain the same. Thus, objects created in assemblies written in other languages of the .NET Framework can be bound to C# variables of any type to which the value can be converted, per the conversion rules below. The following illustrates the cross-language compatibility of types by comparing C# code with the equivalent Visual Basic .NET code:

 // C#
 public void UsingCSharpTypeAlias()
 {
   int i = 42;
 }
 public void EquivalentCodeWithoutAlias()
 {
   System.Int32 i = 42;
 }

 ' Visual Basic .NET
 Public Sub UsingVisualBasicTypeAlias()
   Dim i As Integer = 42
 End Sub
 Public Sub EquivalentCodeWithoutAlias()
   Dim i As System.Int32 = 42
 End Sub

Using the language-specific type aliases is often considered more readable than using the fully-qualified .NET Framework type names.

The fact that each C# type corresponds to a type in the unified type system gives each value type a consistent size across platforms and compilers. That consistency is an important distinction from other languages such as C, where, e.g. a long is only guaranteed to be at least as large as an int, and is implemented with different sizes by different compilers. As reference types, variables of types derived from object (i.e. any class) are exempt from the consistent size requirement. That is, the size of reference types like System.IntPtr, as opposed to value types like System.Int32, may vary by platform. Fortunately, there is rarely a need to know the actual size of a reference type.

There are two predefined reference types: object, an alias for the System.Object class, from which all other reference types derive; and string, an alias for the System.String class. C# likewise has several integral value types, each an alias to a corresponding value type in the System namespace of the .NET Framework. The predefined C# type aliases expose the methods of the underlying .NET Framework types. For example, since the .NET Framework's System.Int32 type implements a ToString() method to convert the value of an integer to its string representation, C#'s int type exposes that method:

 int i = 97;
 string s = i.ToString();
 // The value of s is now the string "97".

Likewise, the System.Int32 type implements the Parse() method, which can therefore be accessed via C#'s int type:

string s = "97";
int i = int.Parse(s);
// The value of i is now the integer 97.

The unified type system is enhanced by the ability to convert value types to reference types (boxing) and likewise to convert certain reference types to their corresponding value types (unboxing). This is also known as casting.

object boxedInteger = 97;
int unboxedInteger = (int)boxedInteger;

Boxing and casting are, however, not type-safe: the compiler won't generate an error if the programmer mixes up the types. In the following short example the mistake is quite obvious, but in complex programs it may be real hard to spot. Avoid boxing, if possible.

object getInteger = "97";
int anInteger = (int)getInteger; // no compile-time error, the program will crash, however

The built-in C# type aliases and their equivalent .NET Framework types follow:

[edit] Integers

[edit] Floating-point

[edit] Other predefined types

[edit] Custom types

The predefined types can be aggregated and extended into custom types.

Custom value types are declared with the struct or enum keyword. Likewise, custom reference types are declared with the class keyword.

[edit] Arrays

Although the number of dimensions is included in array declarations, the size of each dimension is not:

string[] s;

Assignments to an array variable (prior to the variable's usage), however, specify the size of each dimension:

 s = new string[5];

As with other variable types, the declaration and the initialization can be combined:

string[] s = new string[5] ;

It is also important to note that like in Java, arrays are passed by reference, and not passed by value. For example, the following code snippet successfully swaps two elements in an integer array:

static void swap (int[] arr, int i, int j)
{
     int temp = arr[i];
     arr[i] = arr[j];
     arr[j] = temp;
}

[edit] Conversion

Values of a given type may or may not be explicitly or implicitly convertible to other types depending on predefined conversion rules, inheritance structure, and explicit cast definitions.

[edit] Predefined conversions

Many predefined value types have predefined conversions to other predefined value types. If the type conversion is guaranteed not to lose information, the conversion can be implicit (i.e. an explicit cast is not required).

[edit] Inheritance polymorphism

A value can be implicitly converted to any class from which it inherits or interface that it implements. To convert a base class to a class that inherits from it, the conversion must be explicit in order for the conversion statement to compile. Similarly, to convert an interface instance to a class that implements it, the conversion must be explicit in order for the conversion statement to compile. In either case, the runtime environment throws a conversion exception if the value to convert is not an instance of the target type or any of its derived types.

[edit] Scope and extent

The scope and extent of variables is based on their declaration. The scope of parameters and local variables corresponds to the declaring method or statement block, while the scope of fields is associated with the instance or class and is potentially further restricted by the field's access modifiers.

The extent of variables is determined by the runtime environment using implicit reference counting and a complex garbage collection algorithm.

C# operators and their precedence closely resemble the operators in other languages of the C family.

Similar to C++, classes can overload most operators, defining or redefining the behavior of the operators in contexts where the first argument of that operator is an instance of that class, but doing so is often discouraged for clarity.

Following are the built-in behaviors of C# operators.

[edit] Arithmetic

The following arithmetic operators operate on numeric operands (arguments a and b in the "sample usage" below).

[edit] Logical

The following logical operators operate on boolean or integral operands, as noted.

[edit] Bitwise shifting

[edit] Relational

The binary relational operators ==, !=, <, >, <=, and >= are used for relational operations and for type comparisons.

[edit] Assignment

The assignment operators are binary. The most basic is the operator =. Not surprisingly, it assigns the value (or reference) of its second argument to its first argument.

(More technically, the operator = requires for its first (left) argument an expression to which a value can be assigned (an l-value) and for its second (right) argument an expression which can be evaluated (an r-value). That requirement of an assignable expression to its left and a bound expression to its right is the origin of the terms l-value and r-value.)

The first argument of the assignment operator (=) is typically a variable. When that argument has a value type, the assignment operation changes the argument's underlying value. When the first argument is a reference type, the assignment operation changes the reference, so the first argument typically just refers to a different object but the object that it originally referenced does not change (except that it may no longer be referenced and may thus be a candidate for garbage collection).

[edit] Short-hand Assignment

The short-hand assignment operators shortens the common assignment operation of a = a operator b into a operator= b, resulting in less typing and neater syntax.

[edit] Type information

[edit] Pointer manipulation

NOTE: Most C# developers agree that direct manipulation and use of pointers is not recommended in C#. The language has many built-in classes to allow you to do almost any operation you want. C# was built with memory-management in mind and the creation and use of pointers is greatly disruptive to this end. This speaks to the declaration of pointers and the use of pointer notation, not arrays. In fact, a program may only be compiled in "unsafe mode" if it uses pointers.

[edit] Overflow exception control

[edit] Others

There are various ways of grouping sets of data together in C#.

[edit] Enumerations

An enumeration is a data type that enumerates a set of items by assigning to each of them an identifier (a name), while exposing an underlying base type for ordering the elements of the enumeration. The underlying type is int by default, but can be any one of the integral types except for char.

Enumerations are declared as follows:

enum Weekday { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday };

The elements in the above enumeration are then available as constants:

Weekday day = Weekday.Monday;
if (day == Weekday.Tuesday)
{
    Console.WriteLine("Time sure flies by when you program in C#!");
}

If no explicit values are assigned to the enumerated items as the example above, the first element has the value 0, and the successive values are assigned to each subsequent element. However, specific values from the underlying integral type can be assigned to any of the enumerated elements (note that the variable must be type cast in order to access the base type):

enum Age { Infant = 0, Teenager = 13, Adult = 18 };

Age myAge = Age.Teenager;
Console.WriteLine("You become a teenager at an age of {0}.", (int)myAge);

The underlying values of enumerated elements may go unused when the purpose of an enumeration is simply to group a set of items together, e.g., to represent a nation, state, or geographical territory in a more meaningful way than an integer could. Rather than define a group of logically related constants, it is often more readable to use an enumeration.

It may be desirable to create an enumeration with a base type other than int. To do so, specify any integral type besides char as with base class extension syntax after the name of the enumeration, as follows:

enum CardSuit : byte { Hearts, Diamonds, Spades, Clubs };

The enumeration type is also helpful if you need to output the value. By calling the .ToString() method on the enumeration, will output the enumerations name (e.g. CardSuit.Hearts.ToString() will output "Hearts").

[edit] Structs

Structures (keyword struct) are light-weight objects. They are mostly used when only a data container is required for a collection of value type variables. Structs are similar to classes in that they can have constructors, methods, and even implement interfaces, but there are important differences.

• Structs are value types while classes are reference types, which means they behave differently when passed into methods as parameters.
• Structs cannot support inheritance. While structs may appear to be limited with their use, they require less memory and can be less expensive if used in the proper way.
• Structs always have a default constructor, even if you don't want one. Classes allow you to hide the constructor away by using the "private" modifier, whereas structures must have one.

A struct can, for example, be declared like this:

struct Person
{
    public string name;
    public System.DateTime birthDate;
    public int heightInCm;
    public int weightInKg;
}

The Person struct can then be used like this:

Person dana = new Person();
dana.name = "Dana Developer";
dana.birthDate = new DateTime(1974, 7, 18);
dana.heightInCm = 178;
dana.weightInKg = 50;

if (dana.birthDate < DateTime.Now)
{
    Console.WriteLine("Thank goodness! Dana Developer isn't from the future!");
}

It is also possible to provide constructors to structs to make it easier to initialize them:

using System;
struct Person
{
    string name;
    DateTime birthDate;
    int heightInCm;
    int weightInKg;

    public Person(string name, DateTime birthDate, int heightInCm, int weightInKg)
    {
        this.name = name;
        this.birthDate = birthDate;
        this.heightInCm = heightInCm;
        this.weightInKg = weightInKg;
    }
}

public class StructWikiBookSample
{
    public static void Main()
    {
        Person dana = new Person("Dana Developer", new DateTime(1974, 7, 18), 178, 50);
    }
}

There is also an alternative syntax for initializing structs:

struct Person
{
    public string Name;
    public int Height;
    public string Occupation;
}
 
public class StructWikiBookSample2
{
    public static void Main()
    {
        Person john = new Person { Name = "John", Height = 182, Occupation = "Programmer" };
    }
}

Structs are really only used for performance reasons and/or if you intend to it by value. Structs work best when holding a total equal to or less than 16 bytes of data. If in doubt, use classes.

[edit] Arrays

Arrays represent a set of items all belonging to the same type. The declaration itself may use a variable or a constant to define the length of the array. However, an array has a set length and it cannot be changed after declaration.

// an array whose length is defined with a constant
int[] integers = new int[20];
 
int length = 0;
System.Console.Write("How long should the array be? ");
length = int.Parse(System.Console.ReadLine());
// an array whose length is defined with a variable
// this array still can't change length after declaration
double[] doubles = new double[length];

Conditional, iteration, jump, and exception handling statements control a program's flow of execution.

An iteration statement can create a loop using keywords such as do, while, for, foreach, and in.

A jump statement can be used to transfer program control using keywords such as break, continue, return, and yield.

An exception handling statement can be used to handle exceptions using keywords such as throw, try-catch, try-finally, and try-catch-finally.

[edit] Conditional statements

A conditional statement decides whether to execute code based on conditions. The if statement and the switch statement are the two types of conditional statements in C#.

[edit] The if statement

As with most of C#, the if statement has the same syntax as in C, C++, and Java. Thus, it is written in the following form:

if-statement ::= "if" "(" condition ")" if-body ["else" else-body]

condition ::= boolean-expression

if-body ::= statement-or-statement-block

else-body ::= statement-or-statement-block

The if statement evaluates its condition expression to determine whether to execute the if-body. Optionally, an else clause can immediately follow the if body, providing code to execute when the condition is false. Making the else-body another if statement creates the common cascade of if, else if, else if, else if, else statements:

using System;

public class IfStatementSample
{
    public void IfMyNumberIs()
    {
        int myNumber = 5;
        if ( myNumber == 4 )
            Console.WriteLine("This will not be shown because myNumber is not 4.");
        else if( myNumber < 0 )
        {
            Console.WriteLine("This will not be shown because myNumber is not negative.");
        }
        else if( myNumber % 2 == 0 )
            Console.WriteLine("This will not be shown because myNumber is not even.");
        else
        {
            Console.WriteLine("myNumber does not match the coded conditions, so this sentence will be shown!");
        }
    }
}

[edit] The switch statement

The switch statement is similar to the statement from C, C++ and Java.

Unlike C, each case statement must finish with a jump statement (which can be break or goto or return). In other words, C# does not support "fall through" from one case statement to the next (thereby eliminating a common source of unexpected behaviour in C programs). However "stacking" of cases is allowed, as in the example below. If goto is used, it may refer to a case label or the default case (e.g. goto case 0 or goto default).

The default label is optional. If no default case is defined, then the default behaviour is to do nothing.

A simple example:

switch (nCPU)
{
    case 0:
         Console.WriteLine("You don't have a CPU! :-)");
         break;
    case 1:
         Console.WriteLine("Single processor computer");
         break;
    case 2:
         Console.WriteLine("Dual processor computer");
         break;
    // Stacked cases
    case 3:
    case 4:
    case 5:
    case 6:
    case 7:
    case 8:
         Console.WriteLine("A multi processor computer");
         break;
    default:
         Console.WriteLine("A seriously parallel computer");
         break;
}

A nice improvement over the C switch statement is that the switch variable can be a string. For example:

switch (aircraft_ident)
{
    case "C-FESO":
         Console.WriteLine("Rans S6S Coyote");
         break;
    case "C-GJIS":
         Console.WriteLine("Rans S12XL Airaile");
         break;
    default:
         Console.WriteLine("Unknown aircraft");
         break;
}

[edit] Iteration statements

An iteration statement creates a loop of code to execute a variable number of times. The for loop, the do loop, the while loop, and the foreach loop are the iteration statements in C#.

[edit] The do...while loop

The do...while loop likewise has the same syntax as in other languages derived from C. It is written in the following form:

do...while-loop ::= "do" body "while" "(" condition ")"

condition ::= boolean-expression

body ::= statement-or-statement-block

The do...while loop always runs its body once. After its first run, it evaluates its condition to determine whether to run its body again. If the condition is true, the body executes. If the condition evaluates to true again after the body has ran, the body executes again. When the condition evaluates to false, the do...while loop ends.

using System;

public class DoWhileLoopSample
{
    public void PrintValuesFromZeroToTen()
    {
         int number = 0;
         do
         {
              Console.WriteLine(number++.ToString());
         } while(number <= 10);
    }
}

The above code writes the integers from 0 to 10 to the console.

[edit] The for loop

The for loop likewise has the same syntax as in other languages derived from C. It is written in the following form:

for-loop ::= "for" "(" initialization ";" condition ";" iteration ")" body

initialization ::= variable-declaration | list-of-statements

condition ::= boolean-expression

iteration ::= list-of-statements

body ::= statement-or-statement-block

The initialization variable declaration or statements are executed the first time through the for loop, typically to declare and initialize an index variable. The condition expression is evaluated before each pass through the body to determine whether to execute the body. It is often used to test an index variable against some limit. If the condition evaluates to true, the body is executed. The iteration statements are executed after each pass through the body, typically to increment or decrement an index variable.

public class ForLoopSample
{
    public void ForFirst100NaturalNumbers()
    {
        for(int i=0; i<100; i++)
        {
            System.Console.WriteLine(i.ToString());
        }
    }
}

The above code writes the integers from 0 to 99 to the console.

[edit] The foreach loop

The foreach statement is similar to the for statement in that both allow code to iterate over the items of collections, but the foreach statement lacks an iteration index, so it works even with collections that lack indices altogether. It is written in the following form:

foreach-loop ::= "foreach" "(" variable-declaration "in" enumerable-expression ")" body

body ::= statement-or-statement-block

The enumerable-expression is an expression of a type that implements IEnumerable, so it can be an array or a collection. The variable-declaration declares a variable that will be set to the successive elements of the enumerable-expression for each pass through the body. The foreach loop exits when there are no more elements of the enumerable-expression to assign to the variable of the variable-declaration.

public class ForEachSample
{
    public void DoSomethingForEachItem()
    {
        string[] itemsToWrite = {"Alpha", "Bravo", "Charlie"};
        foreach (string item in itemsToWrite)
            System.Console.WriteLine(item);
    }
}

In the above code, the foreach statement iterates over the elements of the string array to write "Alpha", "Bravo", and "Charlie" to the console.

[edit] The while loop

The while loop has the same syntax as in other languages derived from C. It is written in the following form:

while-loop ::= "while" "(" condition ")" body

condition ::= boolean-expression

body ::= statement-or-statement-block

The while loop evaluates its condition to determine whether to run its body. If the condition is true, the body executes. If the condition then evaluates to true again, the body executes again. When the condition evaluates to false, the while loop ends.

using System;

public class WhileLoopSample
{
    public void RunForAwhile()
    {
        TimeSpan durationToRun = new TimeSpan(0, 0, 30);
        DateTime start = DateTime.Now;
        while (DateTime.Now - start < durationToRun)
        {
            Console.WriteLine("not finished yet");
        }
        Console.WriteLine("finished");
    }
}

[edit] Jump statements

A jump statement can be used to transfer program control using keywords such as break, continue, return, yield, and throw.

[edit] break

The break keyword transfers program control to the end of a loop or switch statement.

[edit] continue

The continue keyword transfers program control just before the end of a loop. The condition for the loop is then checked, and if it is met, the loop performs another iteration.

[edit] return

The return keyword identifies the return value for the function or method (if any), and transfers control to the end of the function.

[edit] yield

  * It will preserve local variables of any functions or subroutines ( After the function terminates ).
   
  * Local variables with static scope will achieve the same effect ( Other Lanquages ) ==
       
           (yield will preserve all local variables of the specific function) 
                             vs     
 (Selective variables can be preserved when static is usedin the  declaration of   the "VARIABLE",  You cannot preserve all variables by declaring the function as static.)

       1.   So we can use "yield return" instead of "return"

       2.  "yield break" can be used to discontinue the iterations and still hold the current values.

 The following example shows the usage of the yield keyword:-

          public static int Counter(){
          .... 
          for (int i=5; i<10; ++i) 
               yield return i;
          }
         static void Main(){
             int n, b;
             n=Counter(); // n will be 5
            
             b=Counter(); // b will be 6
      
              //here n=Counter() will be 7 and so on.
              }

[edit] throw

The throw keyword throws an exception. If it is located within a try block, it will transfer control to a catch block that matches the exception - otherwise, it will check if any calling functions are contained within the matching catch block and transfer execution there. If no functions contain a catch block, the program may terminate because of an unhanded exception.

Exceptions and the throw statement are described in greater detail in the Exceptions chapter.

[edit] Background

The exception handling system in the C# language allows the programmer to handle errors or anomalous situations in a structured manner that allows the programmer to separate the normal flow of the code from error-handling logic. An exception can represent a variety of abnormal conditions, including, for example, the use of a null object reference detected by the runtime system, or an invalid input string entered by a user and detected by application code. Code that detects an error condition is said to throw an exception and code that handles the error is said to catch the exception. An exception in C# is an object that encapsulates various information about the error that occurred, such as the stack trace at the point of the exception and a descriptive error message. All exception objects are instantiations of the System.Exception or a child class of it. There are many exception classes defined in the .NET Framework used for various purposes. Programmers may also define their own class inheriting from System.Exception or some other appropriate exception class from the .NET Framework.

Microsoft recommendations prior to version 2.0 recommended that a developer inherit from the ApplicationException exception class. After 2.0 was released, this recommendation was made obsolete and users should inherit from the Exception class^[1].

[edit] Overview

There are 3 code definitions for exception handling. These are:

• try/catch - Do something and catch an error if it should occur.
• try/catch/finally - Do something and catch an error if it should occur, but ALWAYS do the finally.
• try/finally - Do something, but ALWAYS do the finally. Any exception that occurs, will be thrown AFTER finally.

Exceptions are caught from most specifc, to least specific. So for example if you try and access a file that does not exist, the CLR would look for exceptions in the following order:

• FileNotFoundException
• IOException (base class of FileNotFoundException)
• SystemException (base class of IOException)
• Exception (base class of SystemException)

If the exception being thrown does not derive or is not in the list of exceptions to catch, it is thrown up the call stack.

Below are some examples of the different types of exceptions

[edit] Examples

[edit] try/catch

The try/catch performs an operation and should an error occur, will transfer control to the catch block, should there be a valid section to be caught by:

class ExceptionTest
{
     public static void Main(string[] args)
     {
          try
          {
               Console.WriteLine(args[0]);
               Console.WriteLine(args[1]);
               Console.WriteLine(args[2]);
               Console.WriteLine(args[3]);
               Console.WriteLine(args[4]);
          }
          catch (ArgumentOutOfRangeException e)
          {
               Console.WriteLine(e.Message);
          }
     }
}

Here is an example with multiple catches:

class ExceptionTest
{
     public static void Main(string[] args)
     {
          try
          {
               string fileContents = new StreamReader(@"C:\log.txt").ReadToEnd();
          }
          catch (UnauthorizedAccessException e) //Access problems
          {
               Console.WriteLine(e.Message);
          }
          catch (FileNotFoundException e)       //File does not exist
          {
               Console.WriteLine(e.Message);
          }
          catch (IOException e)                //Some other IO problem.
          {
               Console.WriteLine(e.Message);
          }
     }
}

In all catch statements you may omit the type of exception and the exception variable name:

try
{
    int number = 1 / 0;
}
catch (DivideByZeroException)
{
    // DivideByZeroException
}
catch
{
    // some other exception
}

[edit] try/catch/finally

Catching the problem is a good idea, but it can sometimes leave your program in an invalid state. For example, if you open a connection to a database, an error occurs and you throw an exception. Where would you close the connection? In both the try AND exception blocks? Well, problems may occur before the close is carried out.

Therefore, the "finally" statement allows you to cater for the "in all cases do this" circumstance. See the example below:

class ExceptionTest
{
     public static void Main(string[] args)
     {
          SqlConnection sqlConn = null;
 
          try
          {
              sqlConn = new SqlConnection ( /*Connection here*/ );
              sqlConn.Open();
 
              //Various DB things
 
              //Notice you do not need to explicitally close the connection, as .Dispose() does this for you.
          }
          catch (SqlException e)
          {
               Console.WriteLine(e.Message);
          }
          finally
          {
               if(sqlConn != null && sqlConn.State != ConnectionState.Closed)
               {
                   sqlConn.Dispose();
               }
          }
     }
}

Notice that the SqlConnection object is declared outside of the try/catch/finally. The reason is that anything declared in the try/catch cannot be seen by the finally. By declaring it in the previous scope, the finally block is able to access it.

[edit] try/finally

The try/finally block allows you to do the same as above, but instead errors that are thrown are dealt with by the catch (if possible) and then thrown up the call stack.

class ExceptionTest
{
     public static void Main(string[] args)
     {
          SqlConnection sqlConn = null;
 
          try
          {
              SqlConnection sqlConn = new SqlConnection ( /*Connection here*/ );
              sqlConn.Open();
 
              //Various DB bits
          }
          finally
          {
               if(sqlConn != null && sqlConn.State != ConnectionState.Closed)
               {
                   sqlConn.Dispose();
               }
          }
     }
}

[edit] Re-throwing exceptions

Sometimes it is better to throw the error up the call stack for 2 reasons.

1. It is not something you would expect to happen.
2. You are placing extra information into the exception, to help diagnosis.

[edit] How NOT to throw exceptions

Some developers write empty try/catch statements like this:

try
{
      //Do something
}
catch(Exception ex)
{
      //Ignore this here
}

This approach is not recommended. You are swallowing the error and continuing on. If this exception was an OutOfMemoryException or a NullReferenceException, it would not be wise to continue. Therefore you should always catch what you would except to occur, and throw everything else.

Below is another example of how to incorrectly catch exceptions

/* Read the Config file, and return the integer value. If it does not exist, then this is a problem! */
 
try
{
     string value = ConfigurationManager.AppSettings["Timeout"];
 
     if (value == null)
         throw new ConfigurationErrorsException("Timeout value is not in the configuration file.");
}
catch( Exception ex )
{
     //Do nothing!
}

As you can see, the ConfigurationErrorsException will be caught by the catch(Exception) block, but it is being ignored completely! This is bad programming as you are ignoring the error.

Some developers believe you should also use:

try
{
   ..
}
catch( Exception ex )
{
     throw ex;
}

This is incorrect. What is happening is that the CLR will now think that the throw ex; statement is the source of the problem, when the problem is actually in the try section. Therefore NEVER re-throw in this way.

[edit] How to catch exceptions

A better approach would be:

/* Read the Config file, and return the integer value. If it does not exist, then this is a problem! */
 
try
{
     string value = ConfigurationManager.AppSettings["Timeout"];
 
     if (value == null)
         throw new ConfigurationErrorsException("Timeout value is not in the configuration file.");
}
catch( Exception ex )
{
     throw; //<-- Throw the existing problem!
}

The throw; keyword means preserve the exception information and throw it up the call stack.

[edit] Extra information within exceptions

An alternative is to give extra information (maybe local variable information) in addition to the exception. In this case, you wrap the exception within another. You usually use an exception which is as specific to the problem as possible, or create your own if you cannot find out that is not specific enough (or if there is extra information you would wish to include).

public OrderItem LoadItem(string itemNumber)
{
    DataTable dt = null;
 
    try
    {
         if(itemNumber==null)
              throw new ArgumentNullException("Item Number cannot be null","itemNumber");
 
         DataTable dt = DataAccess.OrderItem.Load(itemNumber);
 
         if(dt.Rows == 0)
              return null;
         else if(dt.Rows > 1)
              throw new DuplicateDataException( "Multiple items map to this item.",itemNumber, dt);
 
         OrderItem item = OrderItem.CreateInstanceFromDataRow(dt.Rows[0]);
 
         if(item == null)
              throw new ErrorLoadingException("Error loading Item " + itemNumber, itemNumber, dt.Rows[0]);
    }
    catch(DuplicateDataException dde)
    {
         throw new ErrorLoadingException("OrderItem.LoadItem failed with Item " + itemNumber, dde); // <-- Include dde (as the InnerException) parameter
    }
    catch(Exception ex)
    {
         throw; //<-- We aren't expecting any other problems, so throw them if they occur.
    }
}

[edit] References

1. ↑ [ApplicationException made obsolete]

[edit] Classes

Namespaces are used to provide a "named space" in which your application resides. They're used especially to provide the C# compiler a context for all the named information in your program, such as variable names. Without namespaces, you wouldn't be able to make, e.g., a class named Console, as .NET already uses one in its System namespace. The purpose of namespaces is to solve this problem, and release thousands of names defined in the .NET Framework for your applications to use, along with making it so your application doesn't occupy names for other applications, if your application is intended to be used in conjunction with another. So namespaces exist to resolve ambiguities a compiler wouldn't otherwise be able to do.

Namespaces are easily defined in this way:

 namespace MyApplication
 {
     // The content to reside in the MyApplication namespace is placed here.
 }

There is an entire hierarchy of namespaces provided to you by the .NET Framework, with the System namespace usually being by far the most commonly seen one. Data in a namespace is referred to by using the . operator, such as:

 System.Console.WriteLine("Hello, world!");

This will call the WriteLine method that is a member of the Console class within the System namespace.

By using the using keyword, you explicitly tell the compiler that you'll be using a certain namespace in your program. Since the compiler would then know that, it no longer requires you to type the namespace name(s) for such declared namespaces, as you told it which namespaces it should look in if it couldn't find the data in your application.

So one can then type like this:

 using System;
 
 namespace MyApplication
 {
   class MyClass
   {
     void ShowGreeting()
     {
         Console.WriteLine("Hello, world!"); // note how System is now not required
     }
   }
 }

Namespaces are global, so a namespace in one C# source file, and another with the same name in another source file, will cause the compiler to treat the different named information in these two source files as residing in the same namespace.

[edit] Nested namespaces

Normally, your entire application resides under its own special namespace, often named after your application or project name. Sometimes, companies with an entire product series decide to use nested namespaces though, where the "root" namespace can share the name of the company, and the nested namespaces the respective project names. This can be especially convenient if you're a developer who has made a library with some usual functionality that can be shared across programs. If both the library and your program shared a parent namespace, that one would then not have to be explicitly declared with the using keyword, and still not have to be completely typed out. If your code was open for others to use, third party developers that may use your code would additionally then see that the same company had developed the library and the program. The developer of the library and program would finally also separate all the named information in their product source codes, for fewer headaches especially if common names are used.

To make your application reside in a nested namespace, you can show this in two ways. Either like this:

 namespace CodeWorks
 {
     namespace MyApplication
     {
         // Do stuff
     }
 }

... or like this:

 namespace CodeWorks.MyApplication
 {
     // Do stuff
 }

Both methods are accepted, and are identical in what they do.

As in other object-oriented programming languages, the functionality of a C# program is implemented in one or more classes. The methods and properties of a class contain the code that defines how the class behaves.

C# classes support information hiding by encapsulating functionality in properties and methods and by enabling several types of polymorphism, including subtyping polymorphism via inheritance and parametric polymorphism via generics.

Several types of C# classes can be defined, including instance classes (standard classes that can be instantiated), static classes, and structures.

Classes are defined using the keyword "class" followed by an identifier to name the class. Instances of the class can then be created with the "new" keyword followed by the name of the class. The code below defines a class called Employee with properties Name and Age and with empty methods GetPayCheck() and Work(). It also defines a Sample class that instantiates and uses the Employee class:

public class Employee
{
    private string _name;
    private int _age;

    public string Name
    {
        set { _name = value; }
        get { return _name; }
    }

    public int Age
    {
        set { _age = value; }
        get { return _age; }
    }
       
    public void GetPayCheck()
    {
    }

    public void Work()
    {
    }
}

public class Sample
{
    public static void Main()
    {
        Employee Marissa = new Employee();
        Marissa.Work();
        Marissa.GetPayCheck();
    }
}

[edit] Methods

C# methods are class members containing code. They may have a return value and a list of parameters, as well as a generic type declaration. Like fields, methods can be static (associated with and accessed through the class) or instance (associated with and accessed through an object instance of the class).

[edit] Constructors

A class's constructors control its initialization. A constructor's code executes to initialize an instance of the class when a program requests a new object of the class's type. Constructors often set properties of their classes, but they are not restricted to doing so.

Like other methods, a constructor can have parameters. To create an object using a constructor with parameters, the new command accepts parameters. The code below defines and then instantiates multiple objects of the Employee class, once using the constructor without parameters and once using the version with a parameter:

public class Employee
{
    public Employee()
    {
        System.Console.WriteLine("Constructed without parameters");
    }

    public Employee(string text)
    {
        System.Console.WriteLine(text);
    }
}

public class Sample
{
    public static void Main()
    {
        System.Console.WriteLine("Start");
        Employee Alfred = new Employee();
        Employee Billy  = new Employee("Parameter for construction");
        System.Console.WriteLine("End");
    }
}

Output:

Start
Constructed without parameters
Parameter for construction
End

Constructors can call each other:

public class Employee
{
    public Employee(string text, int number)
    {
        ...
    }
 
    public Employee(string text)
        : this(text, 1234) // calls the above constructor with user-specified text and the default number
    { }
 
    public Employee()
        : this("default text") // calls the above constructor with the default text
    { }
}

[edit] Finalizers

The opposite of constructors, finalizers define the final behavior of an object and execute when the object is no longer in use. Although they are often used in C++ to free memory reserved by an object, they are less frequently used in C# due to the .NET Framework Garbage Collector. An object's finalizer, which takes no parameters, is called sometime after an object is no longer referenced, but the complexities of garbage collection make the specific timing of finalizers uncertain.

public class Employee
{
    public Employee(string text)
    {
        System.Console.WriteLine(text);
    }

    ~Employee()
    {
        System.Console.WriteLine("Finalized!");
    }

    public static void Main()
    {
        Employee Marissa = new Employee("Constructed!");
        Marissa = null;
    }
}

Output:

Constructed!
Finalized!

[edit] Properties

C# properties are class members that expose functionality of methods using the syntax of fields. They simplify the syntax of calling traditional get and set methods (a.k.a. accessor methods). Like methods, they can be static or instance.

Properties are defined in the following way:

public class MyClass
{
    private int integerField = 3; // Sets integerField with a default value of 3

    public int IntegerField
    {
        get {
            return integerField;  // get returns the field you specify when this property is assigned
        }
        set {
            integerField = value; // set assigns the value assigned to the property of the field you specify
        }
    }
}

The C# keyword value contains the value assigned to the property. After a property is defined it can be used like a variable. If you were to write some additional code in the get and set portions of the property it would work like a method and allow you to manipulate the data before it is read or written to the variable.

using System;

public class MyProgram
{
    MyClass myClass = new MyClass;
    
    Console.WriteLine(myClass.IntegerField); // Writes 3 to the command line.
    myClass.IntegerField = 7; // Indirectly assigns 7 to the field myClass.integerField     
}

Using properties in this way provides a clean, easy to use mechanism for protecting data.

[edit] Indexers

C# indexers are class members that define the behavior of the array access operation (e.g. list[0] to access the first element of list even when list is not an array).

To create an indexer, use the this keyword as in the following example:

public string this[string key]
{
   get { return coll[key]; }
   set { coll[key] = value; }
}

This code will create a string indexer that returns a string value. For example, if the class was EmployeeCollection, you could write code similar to the following:

EmployeeCollection e = new EmployeeCollection();
.
.
.
string s = e["Jones"];
e["Smith"] = "xxx";

[edit] Events

C# events are class members that expose notifications to clients of the class.

[edit] Operator overloading

C# operator definitions are class members that define or redefine the behavior of basic C# operators (called implicitly or explicitly) on instances of the class:

public class Complex
{
    private double re, im;
 
    public double Real
    {
        get { return re; }
        set { re = value; }
    }
 
    public double Imaginary
    {
        get { return im; }
        set { im = value; }
    }
 
    // binary operator overloading
    public static Complex operator +(Complex c1, Complex c2)
    {
        return new Complex() { Real = c1.Real + c2.Real, Imaginary = c1.Imaginary + c2.Imaginary };
    }
 
    // unary operator overloading
    public static Complex operator -(Complex c)
    {
        return new Complex() { Real = -c.Real, Imaginary = -c.Imaginary };
    }
 
    // cast operator overloading (both implicit and explicit)
    public static implicit operator double(Complex c)
    {
        // return the modulus - sqrt(x^2 + y^2)
        return Math.Sqrt(Math.Pow(c.Real, 2) + Math.Pow(c.Imaginary, 2));
    }
 
    public static explicit operator string(Complex c)
    {
        // we should be overloading the ToString() method, but this is just a demonstration
        return c.Real.ToString() + " + " + c.Imaginary.ToString() + "i";
    }
}
 
public class StaticDemo
{
    public static void Main()
    {
        Complex number1 = new Complex() { Real = 1, Imaginary = 2 };
        Complex number2 = new Complex() { Real = 4, Imaginary = 10 };
        Complex number3 = number1 + number2; // number3 now has Real = 5, Imaginary = 12
 
        number3 = -number3; // number3 now has Real = -5, Imaginary = -12
        double testNumber = number3; // testNumber will be set to the absolute value of number3
        Console.WriteLine((string)number3); // This will print "-5 + -12i".
        // The cast to string was needed because that was an explicit cast operator.
    }
}

[edit] Structures

Structures, or structs, are defined with the struct keyword followed by an identifier to name the structure. They are similar to classes, but have subtle differences. Structs are used as lightweight versions of classes that can help reduce memory management efforts when working with small data structures. In most situations, however, using a standard class is a better choice.

The principal difference between structs and classes is that instances of structs are values whereas instances of classes are references. Thus when you pass a struct to a function by value you get a copy of the object so changes to it are not reflected in the original because there are now two distinct objects but if you pass an instance of a class by value then there is only one instance.

The Employee structure below declares a public and a private field. Access to the private field is granted through the public property "Name":

struct Employee
{
    private string name;
    public int age;

    public string Name
    {
        set { name = value; }
        get { return name; }
    }
}

[edit] Static classes

Static classes are commonly used to implement a Singleton Pattern. All of the methods, properties, and fields of a static class are also static (like the WriteLine() method of the System.Console class) and can thus be used without instantiating the static class:

public static class Writer
{
    public static void Write()
    {
        System.Console.WriteLine("Text");
    }
}

public class Sample
{
    public static void Main()
    {
        Writer.Write();
    }
}

[edit] Introduction

The .NET framework consists of several languages, all which follow the "object orientated programming" (OOP) approach to software development. This standard defines that all objects support

• Inheritance - the ability to inherit and extend existing functionality.
• Encapsulation - the ability to allow the user to only see specific parts, and to interact with it in specific ways.
• Polymorphism - the ability for an object to be assigned dynamically, but with some predictability as to what can be done with the object.

Objects are synonymous with objects in the real world. Think of any object and think of how it looks and how it is measured and interacted with. When creating OOP languages, the reasoning was that if it mimics the thought process of humans, it would simplify the coding experience.

For example, let's consider a chair, and its dimensions, weight, colour and what is it made out of. In .NET, these values are called "Properties". These are values which define the object's state. Be careful, as there are two ways to expose values: Variables and Properties. The recommended approach is expose Properties and not variables.

So we have a real-world idea of the concept of an object. In terms of practicality for a computer to pass information about, passing around an object within a program would consume a lot of resources. Think of a car, how many properties that has - 100's, 1000's. A computer passing this information about all the time will waste memory, processing time and therefore a bad idea to use. So objects come in 2 flavours:

• Reference types
• Value types

[edit] Reference and Value Types

A reference type is like a pointer to the value. Think of it like a piece of paper with a street address on it, and the address leads to your house - your object with hundreds of properties. If you want to find it, go to where the address says! This is exactly what happens inside the computer. The reference is stored as a number, corresponding to somewhere in memory where the object exists. So instead of moving an object around - like building a replica house every time you want to look at it - you just look at the original.

A value type is the exact value itself. Values are great for storing small amounts of information: numbers, dates etc.

There are differences in the way they are processed, so we will leave that until a little later in the article.

As well as querying values, we need a way to interact with the object so that some operation can be performed. Think of files - its all well and good knowing the length of the file, but how about Read()'ing it? Therefore, we can use something called methods as a way of performing actions on an object.

An example would be a rectangle. The properties of a rectangle are:

• Length
• Width

The "functions" (or methods in .NET) would be:

• Area ( = Length * Width )
• Perimeter ( = 2 * Length + 2 * Width)

Methods vary from Properties because they require some transformation of data to achieve a result. Methods can either return a result (such as Area) or not. Like above with the chair, if you Sit() on the chair, there is no expected reaction, the chair just ... works!

[edit] System.Object

To support the first rule of OOP - Inheritance, we define something that all objects will derive from - this is System.Object, also known as Object or object. This object defines some methods that all objects can use should they need to. These methods include:

• GetHashCode() - retrieve a number unique to that object.
• GetType() - retrieves information about the object like method names, the objects name etc.
• ToString() - convert the object to a textual representation - usually for outputting to the screen or file.

Since all objects derive from this class (whether you define it or not), any class will have these 3 methods ready to use. Since we always inherit from System.Object, or a class that itself inherits from System.Object, we therefore enhance and/or extend its functionality. Like in the real world that humans, cats, dogs, birds, fish are all an improved and specialised version of an "organism".

[edit] Object basics

All objects by default are reference types. To support value types, objects must instead inherit from the System.ValueType abstract class, rather than System.Object.

[edit] Constructors

When objects are created, they are initialized by the "constructor". The constructor sets up the object, ready for use. Because objects need to be created before being used, the constructor is created implicitly, unless it is defined differently by the developer. There are 3 types of constructor:

• Static Constructor
• Default constructor - takes no parameters.
• Overloaded constructor - takes parameters.

Overloaded constructors automatically remove the implicit default constructor, so a developer must explicitly define the default constructor if they want to use it.

[edit] Static Constructor

A static constructor is first called when the runtime first accesses the class. Static variables are accessible at all times, so the runtime must initialize it on its first access. The example below, when stepping through in a debugger, will show that static MyClass() is only accessed when the MyClass.Number variable is accessed.

using System;
using System.Collections.Generic;
using System.Text;
 
namespace StaticConstructors
{
    class Program
    {
        static void Main(string[] args)
        {
            int i = 0;
            int j = 0;
            Console.WriteLine("Static Number = " + MyClass.Number);
        }
    }
 
    class MyClass
    {
        private static int number;
        public static int Number { get { return number; } }
        static MyClass()
        {
            Random r = new Random();
            number = r.Next();
        }
    }
}

[edit] Default Constructor

The default constructor takes no parameters and is implicitly defined if no other constructors exist. The code sample below show the before, and after result of creating a class.

//Created by the developer
class MyClass
{
}
 
//Created by the compiler
class MyClass : System.Object
{
     public MyClass() : base()
     {
     }
}

[edit] Overloaded Constructors

To initialize objects in various forms, the constructors allow customization of the object by passing in parameters.

 class MyClass
    {
        private int number;
        public int Number { get { return number; } }
 
        public MyClass()
        {
            Random r = new Random();
            number = r.Next();
        }
 
        public MyClass(int seed)
        {
            Random r = new Random(seed);
            number = r.Next();
        }
   }

[edit] Calling other constructors

To minimise code, if another constructor implements the functionality better, you can instruct the constructor to call an overloaded (or default) constructor with specific parameters.

 class MyClass
    {
        private int number;
        public int Number { get { return number; } }
 
        public MyClass() : 
             this ( DateTime.Now.Milliseconds ) //Call the other constructor passing in a value.
        {
 
        }
 
        public MyClass(int seed)
        {
            Random r = new Random(seed);
            number = r.Next();
        }
   }

Base classes constructors can also be called instead of constructors in the current instance

 class MyException : Exception
    {
        private int number;
        public int Number { get { return number; } }
 
        public MyException ( int errorNumber, string message, Exception innerException) : base( message, innerException )
        {
             number = errorNumber;
        }
   }

[edit] Destructors

As well as being "constructed", objects can also perform cleanup when they are cleared up by the garbage collector. The garbage collector only runs when either directly invoked, or has reason to reclaim memory, therefore the destructor may not get the chance to clean up resources for a long time. In this case, look into use of the Dispose() method, from the IDisposable interface.

Destructors are recognised via the use of the ~ symbol in front of a constructor with no access modifier e.g.

 class MyException : Exception
    {
        private int number;
        public int Number { get { return number; } }
 
        public MyException ( int errorNumber, string message, Exception innerException) : base( message, innerException )
        {
             number = errorNumber;
        }
 
        ~MyException()
        {
        }
   }

Encapsulation is depriving of the user of a class information he does not need, and preventing him from manipulating objects in ways not intended by the designer.

A class element having public protection level is accessible to all code anywhere in the program. These methods and properties represent the operations allowed on the class to outside users.

Methods, data members (and other elements) with private protection level represent the internal state of the class (for variables), and operations which are not allowed to outside users. The private protection level is default for all class and struct members. This means that if you do not specify the protection modifier of a method or variable, it is considered as private by the compiler.

For example:

public class Frog
{
	public void JumpLow() { Jump(1); }
	public void JumpHigh() { Jump(10); }

	void Jump(int height) { _height += height;}

	private int _height = 0;
}

In this example, the public method the Frog class exposes are JumpLow and JumpHigh. Internally, they are implemented using the private Jump function that can jump to any height. This operation is not visible to an outside user, so he cannot make the frog jump 100 meters, only 10 or 1. The Jump private method is implemented by changing the value of a private data member _height, which is also not visible to an outside user. Some private data members are made visible by Properties.

[edit] Protection Levels

[edit] Private

Private members are only accessible within the class itself. A method in another class, even a class derived from the class with private members cannot access the members.

[edit] Protected

Protected members can be accessed by the class itself and by any class derived from that class.

[edit] Public

Public members can be accessed by any method in any class.

[edit] Internal

Internal members are accessible only in the same assembly and invisible outside it.

If no protection level is specified, class members are usually treated as internal. However, nested classes or types will have a different default protection level.

[edit] Protected Internal

Protected internal members are accessible from any class derived from the that class, or any class within the same assembly.

[edit] Advanced Concepts

[edit] Inheritance

Inheritance is the ability to create a class from another class, the "parent" class, extending the functionality and state of the parent in the derived, or "child", class.

Inheritance in C# also allows derived classes to overload methods from their parent class.

[edit] Subtyping Inheritance

The code sample below shows two classes, Employee and Executive. Employee has the following methods, GetPayCheck and Work.

We want the Executive class to have the same methods, but differently implemented and one extra method, AdministerEmployee.

Below is the creation of the first class to be derived from, Employee.

public class Employee
   {
       // we declare one method virtual so that the Executive class can
       // override it.
       public virtual void GetPayCheck()
       {
           //get paycheck logic here.
       }
 
       //Employee's and Executives both work, so no virtual here needed.
       public void Work()
       {
           //do work logic here.
       }
   }

Now, we create an Executive class that will override the GetPayCheck method.

public class Executive : Employee
   {
       // the override keyword indicates we want new logic behind the GetPayCheck method.
       public override void GetPayCheck()
       {
           //new getpaycheck logic here.
       }
   
       // the extra method is implemented.
       public void AdministerEmployee()
       {
           // manage employee logic here
       }
   }

You'll notice that there is no Work method in the Executive class, it is not required, since that method is automatically added to the Executive class, because it derives its methods from Employee, which has the Work method.

static void Main(string[] args)
       {
            Employee emp = new Employee;
            Executive exec = new Executive;
       
            emp.Work();
            exec.Work();
            emp.GetPayCheck();
            exec.GetPayCheck();
            exec.AdministerEmployee();
       }

[edit] Virtual Methods

If a base class contains a virtual method which it calls elsewhere and a derived class overrides that virtual method, the base class will actually call the derived class' method:

public class Resource : IDisposable
{
    private bool closed;
 
    protected virtual void Close()
    {
        Console.WriteLine("Base resource closer called!");
    }
 
    ~Resource()
    {
        Dispose();
    }
 
    public void Dispose()
    {
        if (!closed)
        {
            Console.WriteLine("Disposing resource and calling the Close() method...");
            closed = true;
            Close();
        }
    }
}
 
public class AnotherTypeOfResource : Resource
{
    protected override void Close()
    {
        Console.WriteLine("Another type of resource closer called!");
    }
}
 
public class VirtualMethodDemo
{
    public static void Main()
    {
        Resource res = new Resource();
        AnotherTypeOfResource res2 = new AnotherTypeOfResource();
 
        res.Dispose(); // Resource.Close() will be called.
        res2.Dispose(); // even though Dispose() is part of the Resource class, 
        // the Resource class will call AnotherTypeOfResource.Close()!
    }
}

[edit] Constructors

A derived class does not automatically inherit the base class' constructors; the derived class cannot be instantiated unless it provides its own. A derived class must call one of its base class' constructors by using the base keyword:

public MyBaseClass
{
    public MyBaseClass(string text)
    {
        ...
    }
}
 
public MyDerivedClass : MyBaseClass
{
    public MyDerivedClass(int number)
        : base(number.ToString())
    { }
 
    public MyDerivedClass(string text) // even though this is exactly the same as MyBaseClass' only constructor, 
    // this is still necessary as constructors are not inherited.
        : base(text)
    { }
}

[edit] Inheritance keywords

How C# inherits from another class syntacticaly is using the ":" operator.

Example. public class Executive : Employee

To indicate a method that can be overridden, you mark the method with virtual.

public virtual void Write(string text)
{
   System.Console.WriteLine("Text:{0}", text);
}

To override a method use the override keyword

public override void Write(string  text)
{
   System.Console.WriteLine(text);
}

An interface in C# is type definition similar to a class except that it purely represents a contract between an object and a user of the object. An interface cannot be directly instantiated as an object. No data members can be defined in an interface. Methods and properties can only be declared, not defined. For example, the following defines a simple interface:

 interface IShape
 {
   void Draw();
   double X { get; set; }
   double Y { get; set; }
 }

A convention used in the .NET Framework (and likewise by many C# programmers) is to place an "I" at the beginning of an interface name to distinguish it from a class name. Another common interface naming convention is used when an interface declares only one key method, such Draw() in the above example. The interface name is then formed by adding the suffix "able" to the method name. So, in the above example, the interface name would be IDrawable. This convention is also used throughout the .NET Framework.

Implementing an interface is simply done by inheriting off the interface and then defining all the methods and properties declared by the interface. For example:

class Square : IShape
{
     private double mX, mY;

     public void Draw() { ... }

     public double X 
     { 
          set { mX = value; }
          get { return mX; }  
     }

     public double Y 
     {
          set { mY = value; }
          get { return mY; }
     }
}

Although a class can only inherit from one other class, it can inherit from any number of interfaces. This is simplified form of multiple inheritance supported by C#. When inheriting from a class and one or more interfaces, the base class should be provided first in the inheritance list followed by any interfaces to be implemented. For example:

class MyClass : Class1, Interface1, Interface2 { ... }

Object references can be declared using an interface type. For example, using the previous examples:

class MyClass 
{
     static void Main()
     {
          IShape shape = new Square();
          shape.Draw();
     }
}

Interfaces can inherit off of any number of other interfaces but cannot inherit from classes. For example:

interface IRotateable
{
     void Rotate(double theta);
}

interface IDrawable : IRotateable
{
     void Draw();
}

[edit] Additional Details

Access specifiers (i.e. private, internal, etc) cannot be provided for interface members. All members are public. A class implementing an interface must define all the members declared by the interface as public. The implementing class has the option of making an implemented method virtual if it is expected to be overridden in a child class.

There are no static methods within an interface. Any static methods can be implemented in a class that manages objects using that interface.

In addition to methods and properties, interfaces can declare events and indexers as well.

[edit] Introduction

Events and delegates are fundamental to any Windows or Web Application. These allow the developer to "subscribe" to particular actions carried out by the user. Therefore instead of expecting everything and filtering out what you want, you choose what you want to be notified of and react to that action.

A delegate is a way of telling C# which method to call when an event is triggered. For example, if you click a Button on a form, the program would call a specific method. It is this pointer which is a delegate. Delegates are good because you can notify several methods that an event has occurred, if you so wish.

An event is a notification by the .NET framework that an action has occurred. Each event contains information about the specific event, e.g., a mouse click would say which mouse button was clicked and where on the form it was clicked.

Lets say you write a program that only reacts to a Button click, here is the sequence of events that occurs:

• User presses the mouse down on a button
  • The .NET framework raises a MouseDown event
• User releases the mouse button
  • The .NET framework raises a MouseUp event
  • The .NET framework raises a MouseClick event
  • The .NET framework raises a Clicked event on the Button

Since the button's click event has been subscribed, the rest of the events are ignored by the program and your delegate tells the .NET framework which method to call, now that the event has been raised.

[edit] Delegates

Delegates are a construct for abstracting and creating objects that reference methods and can be used to call those methods. Delegates form the basis of event handling in C#. A delegate declaration specifies a particular method signature. References to one or more methods can be added to a delegate instance. The delegate instance can then be "called" which effectively calls all the methods that have been added to the delegate instance. A simple example:

using System;
delegate void Procedure();

class DelegateDemo
{	
    static void Method1()
    {
         Console.WriteLine("Method 1");
    }

    static void Method2()
    {
         Console.WriteLine("Method 2");
    }

    void Method3()
    {
         Console.WriteLine("Method 3");
    }

    static void Main()
    {
         Procedure someProcs = null;
         someProcs += new Procedure(DelegateDemo.Method1);
         someProcs += new Procedure(DelegateDemo.Method2);
         DelegateDemo demo = new DelegateDemo();
         someProcs += new Procedure(demo.Method3);
         someProcs();
    }
}

In this example, the delegate is declared by the line delegate void Procedure(); This statement is a complete abstraction. It does not result in executable code that does any work. It merely declares a delegate type called Procedure which takes no arguments and returns nothing. Next, in the Main() method, the statement Procedure someProcs = null; instantiates a delegate. Something concrete has now been created. The assignment of null to someProcs means that the delegate is not initially referencing any methods. The statements someProcs += new Procedure(DelegateDemo.Method1); and someProcs += new Procedure(DelegateDemo.Method2); add two static methods to the delegate instance. (Note: the class name could have been left off of DelegateDemo.Method1 and DelegateDemo.Method2 because the statement is occurring in the DelegateDemo class.) The statement someProcs += new Procedure(demo.Method3); adds a non-static method to the delegate instance. For a non-static method, the method name is preceded by an object reference. When the delegate instance is called, Method3() is called on the object that was supplied when the method was added to the delegate instance. Finally, the statement someProcs(); calls the delegate instance. All the methods that were added to the delegate instance are now called in the order that they were added.

Methods that have been added to a delegate instance can be removed with the -= operator:

 someProcess -= new Procedure(DelegateDemo.Method1);

In C# 2.0, adding or removing a method reference to a delegate instance can be shortened as follows:

    someProcess += DelegateDemo.Method1;
    someProcess -= DelegateDemo.Method1;

Invoking a delegate instance that presently contains no method references results in a NullReferenceException.

Note that if a delegate declaration specifies a return type and multiple methods are added to a delegate instance, then an invocation of the delegate instance returns the return value of the last method referenced. The return values of the other methods cannot be retrieved (unless explicitly stored somewhere in addition to being returned).

[edit] Anonymous Delegates

Anonymous delegates are a short way to write delegate code, specified using the delegate keyword. The delegate code can also reference local variables of the function in which they are declared. Anonymous delegates are automatically converted into methods by the compiler. For example:

using System;
delegate void Procedure();
 
class DelegateDemo2
{
    static Procedure someProcs = null;
 
    private static void AddProc()
    {
        int variable = 100;
 
        someProcs += new Procedure(delegate
            {
               Console.WriteLine(variable);
            });
    }
 
    static void Main()
    {
        someProcs += new Procedure(delegate { Console.WriteLine("test"); });
        AddProc();
        someProcs();
        Console.ReadKey();
    }
}

They can accept arguments just as normal methods can:

using System;
delegate void Procedure(string text);
 
class DelegateDemo3
{
    static Procedure someProcs = null;
 
    private static void AddProc()
    {
        int variable = 100;
 
        someProcs += new Procedure(delegate(string text)
            {
               Console.WriteLine(text + ", " + variable.ToString());
            });
    }
 
    static void Main()
    {
        someProcs += new Procedure(delegate(string text) { Console.WriteLine(text); });
        AddProc();
        someProcs("testing");
        Console.ReadKey();
    }
}

The output is:

testing
testing, 100

[edit] Lambda Expressions

Lambda expressions are a clearer way to achieve the same thing as an anonymous delegate. Its form is:

(type1 arg1, type2 arg2, ...) => expression

This is equivalent to:

delegate(type1 arg1, type2 arg2, ...)
{
    return expression;
}

If there is only one argument, the parentheses can be omitted. The type names can also be omitted to let the compiler infer the types from the context. In the following example, str is a string and the return type is a int:

Func<string, int> myFunc = str => int.Parse(str);

This is equivalent to:

Func<string, int> myFunc = delegate(string str)
{
    return int.Parse(str);
};

[edit] Events

An event is a special kind of delegate that facilitates event-driven programming. Events are class members which cannot be called outside of the class regardless of its access specifier. So, for example, an event declared to be public would allow other classes the use of += and -= on the event, but firing the event (i.e. invoking the delegate) is only allowed in the class containing the event. A simple example:

delegate void ButtonClickedHandler();

class Button
{
    public event ButtonClickedHandler ButtonClicked;
				
    public void SimulateClick()
    {
         if (ButtonClicked != null)
         {
              ButtonClicked();
         }
    }

    ...

}

A method in another class can then subscribe to the event by adding one of its methods to the event delegate:

Button b = new Button();
b.ButtonClicked += MyHandler;

Even though the event is declared public, it cannot be directly fired anywhere except in the class containing the event.

In general terms, an interface is the set of public members of a component. Of course, a C# interface is an interface that defines a set of public members. A C# class also defines an interface because it has a set of public members. A nonabstract C# class also defines the implementation of each member.

In C# it is possible to have a type that is intermediate between a pure interface that does not define any implementation, and a type that defines a complete implementation. This is called an abstract class.

You define an abstract class by including the abstract keyword on the class definition.

An abstract class is somewhere between a C# interface and a nonabstract class. Of the public members defined by an abstract class, any number of those members may include an implementation.

For example, an abstract class might provide an implementation for none of its members.

public abstract class AbstractShape
{
   public abstract void Draw(Graphics g);
   public abstract double X {get; set;}
   public abstract double Y {get; set;}
}

This class is equivalent to an interface in many respects. (One difference is that a class that derives from this class cannot derive from any other class.)

An abstract class may also define all of its members.

public abstract class AbstractShape
{
   private double x;
   private double y;
   //
   // ... (other members)
   //
   public void Draw(Graphics g) {g.DrawRectangle(Pens.Black, g_rect);}
   public double X {get{return x;}}
   public double Y {get{return y;}}
}

And an abstract class may define some of its members but leave others undefined.

public abstract class AbstractShape
{
   private double x;
   private double y;
   //
   // ... (other members)
   //
   public abstract void Draw(Graphics g);
   public double X {get{return x;}}
   public double Y {get{return y;}}
}

An abstract class is similar to a nonabstract class, but there are some important differences.

For one thing, you cannot create an instance of an abstract class with the new keyword. For example, the following statement will raise a compiler error:

   AbstractShape shape = new AbstractShape();

Of course, assuming the concrete class Square derives from AbstractShape, the following would be correct:

   AbstractShape shape = new Square();

A second difference is that an abstract class can contain abstract members. As was shown above, it does not have to contain abstract members. The point is that a nonabstract class may not contain abstract members. That is, you must include the abstract keyword on the class if you include even one abstract member.

The third difference is that an abstract class cannot be sealed. That is, you cannot use both the abstract keyword and the sealed keyword on the same class.

[edit] Implementing methods

As with virtual methods, you can implement abstract methods or properties with the override keyword:

public class Rectangle : AbstractShape
{
    private double x;
    private double y;
    // ...
    public override void Draw(Graphics g)
    {
        g.DrawRectangle(Pens.Black, g_rect);
    }
 
    public override double X {
        get { return x; }
        set { x = value; }
    }
 
    public override double Y {
        get { return x; }
        set { x = value; }
    }
}

Overriding an abstract method is effectivly the same as overriding a virtual method - you cannot change the access specifiers (i.e. you can't convert an protected abstract method into public), and you cannot add a missing get or set to an abstract property.

[edit] Partial Classes

As the name indicates, partial class definitions can be split up across multiple physical files. To the compiler, this does not make a difference as all the fragments of the partial class are grouped and the compiler treats it as a single class. One common usage of partial classes is the separation of automatically generated code from programmer written code.

Below is the example of a partial class.

Listing 1: Entire class definition in one file (file1.cs)

public class Node
{
    public bool Delete()
    {
    }

    public bool Create()
    {
    }
}

Listing 2: Class split across multiple files

(file1.cs)

public partial class Node
{
    public bool Delete()
    {
    }
}

(file2.cs)

public partial class Node
{
    public bool Create()
    {
    }
}

Generics are a new feature in version 2.0 of the C# language and the common language runtime (CLR). Generics introduce to the .NET Framework the concept of type parameters, which make it possible to design classes and methods that defer the specification of one or more types until the class or method is declared and instantiated by client code. The most common use of generics is to create collection classes. Generic types were introduced to maximize code reuse, type safety, and performance.^[1]

[edit] Generic Classes

There are cases when you need to create a class to manage objects of some type, without modifying them. Without Generics, the usual approach (highly simplified) to make such class would be like this:

public class SomeObjectContainer
{
    private object obj;
 
    public SomeObjectContainer(object obj)
    {
        this.obj = obj;
    }
 
    public object GetObject()
    {
        return this.obj;
    }
}

And its usage would be:

class Program
{
    static void Main(string[] args)
    {
        SomeObjectContainer container = new SomeObjectContainer(25);
        SomeObjectContainer container2 = new SomeObjectContainer(5);
        Console.WriteLine((int)container.GetObject() + (int)container2.GetObject());
 
        Console.ReadKey(); // wait for user to press any key, so we could see results
    }
}

Notice that we have to cast back to original data type we have chosen (in this case - int) every time we want to get an object from such a container. In such small programs like this everything is clear. But in more complicated cases with more containers in different parts of the program, we would have to take care that the container is supposed to have int type in it, would not have a string or any other data type. If that happens, InvalidCastException is thrown.

Additionally, if the original data type we have chosen is a value type, such as int, we will incur a performance penalty every time we access the elements of the collection, due to the Autoboxing feature of C#.

However, we could surround every unsafe area with try - catch block, or we could create a separate "container" for every data type we need, just to avoid casting. While both ways could work (and worked for many years), it is unnecessary now, because Generics offers a much more elegant solution.

To make our "container" class to support any object and avoid casting, we replace every previous object type with some new name, in this case - T, and add <T> mark immediately after the class name to indicate that this "T" type is Generic / any type.

Note: You can choose any name and use more than one generic type for class, i.e <genKey, genVal>

public class GenericObjectContainer<T>
{
    private T obj;
 
    public GenericObjectContainer(T obj)
    {
        this.obj = obj;
    }
 
    public T getObject()
    {
        return this.obj;
    }
}

Not a big difference, which results in simple and safe usage:

class Program
{
    static void Main(string[] args)
    {
        GenericObjectContainer<int> container = new GenericObjectContainer<int>(25);
        GenericObjectContainer<int> container2 = new GenericObjectContainer<int>(5);
        Console.WriteLine(container.getObject() + container2.getObject());
 
        Console.ReadKey(); // wait for user to press any key, so we could see results
    }
}

Generics ensures that you specify the type for a "container" only when creating it, and after that you will be able to use only the type you specified. But now you can create containers for different object types, and avoid the previously mentioned problems. In addition, this avoids the Autoboxing for struct types.

While this example is far from practical, it does illustrate some situations where generics are useful:

• You need to keep objects of a single type in a class
• You don't need to modify objects
• You need to manipulate objects in some way
• You wish to store a "value type" (such as int, short, string, or any custom struct) in a collection class without incurring the performance penalty of Autoboxing every time you manipulate the stored elements.

[edit] Generic Interfaces

A generic interface is an interface that accepts one or more type parameters, similar to a generic class:

public interface IContainer<T>
{
    T GetObject();
    void SetObject(T value);
}
 
public class StringContainer : IContainer<string>
{
    private string str;
 
    public string GetObject()
    {
        return str;
    }
 
    public void SetObject(string value)
    {
        str = value;
    }
}
 
public class FileWithString : IContainer<string>
{
    ...
}

class Program
{
    static void Main(string[] args)
    {
        IContainer<string> container = new StringContainer();
 
        container.SetObject("test");
        Console.WriteLine(container.GetObject());
        container = new FileWithString();
        container.SetObject("another test");
        Console.WriteLine(container.GetObject());
 
        Console.ReadKey();
    }
}

Generic interfaces are useful when we could have multiple implementations of a particular generic class. For example, both the List<T> class (discussed below) and the LinkedList<T> class implement the IEnumerable<T> interface. List<T> has a constructor that creates a new list based on an existing object that implements IEnumerable<T>, and so we can write the following:

LinkedList<int> linkedList = new LinkedList<int>();
 
linkedList.AddLast(1);
linkedList.AddLast(2);
linkedList.AddLast(3);
// linkedList now contains 1, 2 and 3.
 
List<int> list = new List<int>(linkedList);
 
// now list contains 1, 2 and 3 as well!

[edit] Generic Methods

Generic methods are very similar to generic classes and interfaces:

public static bool ArrayContains<T>(T[] array, T element)
{
    foreach (T e in array)
    {
        if (e.Equals(element))
        {
            return true;
        }
    }
    return false;
}

This method can be used to search any type of array:

class Program
{
    static void Main(string[] args)
    {
        string[] strArray = { "string one", "string two", "string three" };
        int[] intArray = { 123, 456, 789 };
 
        Console.WriteLine(ArrayContains<string>(strArray, "string one")); // True
        Console.WriteLine(ArrayContains<int>(intArray, 135)); // False
    }
}

[edit] Type Constraints

You may specify one or more type constraints in any generic class, interface or method using the where keyword. The following example shows all of the possible type constraints:

public class MyClass<T, U, V, W>
    where T : class, // T must be a reference type (class, interface, delegate, array)
    new() // T must have a public constructor with no parameters
    where U : struct // U must be a value type (struct, int, long, float, double, byte, uint, etc.)
    where V : MyOtherClass, // V must be derived from MyOtherClass
    IEnumerable<U> // V must implement IEnumerable<U>
    where W : T, // W must be derived from T
    IDisposable // W must implement IDisposable
{
    ...
}

These type constraints are often necessary

1. To create a new instance of a generic type (the new()) constraint
2. To use foreach on a variable of a generic type (the IEnumerable<T> constraint)
3. To use using on a variable of a generic type (the IDisposable constraint)

[edit] Notes

Extension methods are a feature new to C# 3.0, and allow you to extend existing types with your own methods. While they are static methods, they are used as if they are normal methods of the class being extended; thus new functionality can be added to an existing class without needing to change or recompile the class itself. However, since they are not directly part of the class, extensions cannot access private or protected methods, properties or fields.

Extension methods must be created inside a static class:

public static class MyExtensions
{
    ...
}

The extension methods themselves must be static, and must contain at least one parameter, the first of which must have the this keyword:

public static class MyExtensions
{
    public static string[] ToStringArray<T>(this List<T> list)
    {
        string[] array = new string[list.Count];
 
        for (int i = 0; i < list.Count; i++)
            array[i] = list[i].ToString();
 
        return array;
    }
}

The type of the first parameter (in this case List<T>) specifies the type on which the extension method will be available. You can now call the extension method like this:

List<int> list = new List<int>();
 
list.Add(1);
list.Add(2);
list.Add(3);
 
string[] strArray = list.ToStringArray(); // strArray will now contain "1", "2" and "3".

Here is the full program:

using System;
using System.Collections.Generic;
 
public static class MyExtensions
{
    public static string[] ToStringArray<T>(this List<T> list)
    {
        string[] array = new string[list.Count];
 
        for (int i = 0; i < list.Count; i++)
            array[i] = list[i].ToString();
 
        return array;
    }
 
    public static void WriteToConsole(this string str)
    {
        Console.WriteLine(str);
    }
 
    public static string Repeat(this string str, int times)
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
 
        for (int i = 0; i < times; i++)
            sb.Append(str);
 
        return sb.ToString();
    }
}
 
class ExtensionMethodsDemo
{
    static void Main()
    {
        List<int> myList = new List<int>();
 
        for (int i = 1; i <= 10; i++)
            myList.Add(i);
 
        string[] myStringArray = myList.ToStringArray();
 
        foreach (string s in myStringArray)
            s.Repeat(4).WriteToConsole();
 
        Console.ReadKey();
    }
}

Note that extension methods can take parameters simply by defining more than one parameter without the this keyword.

[edit] Introduction

All computer programs use up memory, whether that is a variable in memory, opening a file or connecting to a database. The question is how can the runtime environment reclaim any memory when it is not being used? There are 3 answers to this question:

• If you are using a managed resource, this is automatically released by the Garbage Collector
• If you are using an unmanaged resource, you must use the IDisposable interface to assist with the cleanup
• If you are calling the Garbage Collector directly, by using System.GC.Collect() method, it will be forced to tidy up resources immediately.

Before discussing managed and unmanaged resources, it would be interesting to know what the garbage collector actually does.

[edit] Garbage Collector

The garbage collector is a background process running within your program. It is always present within all .NET applications. Its job is to look for objects (i.e. reference types) which are no longer being used by your program. If the object is assigned to null, or the object goes out of scope, the garbage collector will mark the object be cleaned up at some point in the future, and not necessarily have its resources released immediately!

Why? The garbage collector will have a hard time keeping up with every de-allocation you make, especially at the speed the program runs and therefore only runs when resources become limited. Therefore, the garbage collector has 3 "generations".

• Generation 0 - the most recently created objects
• Generation 1 - the mid-life objects
• Generation 2 - the long term objects.

All reference types will exist in one of these 3 generations. They will firstly be allocated to Gen 0, then moved to Gen 1 and Gen 2 depending on their lifetime. The garbage collector works by removing only what is needed and so will only scan Gen 0 for a quick-fix solution. This is because most if not all local variables are placed in this area.

For more in-depth information, visit the MSDN Article for a better explanation.

Now you know about the garbage collector, lets discuss the resources that it is managing.

[edit] Managed Resources

Managed resources are objects which run totally within the .NET framework. All memory is reclaimed for you automatically, all resources closed and you are in most cases guaranteed to have all the memory released after the application closes, or when the garbage collector runs.

You do not have to do anything with them with regards to closing connections or anything, it is a self-tidying object.

[edit] Unmanaged Resources

There are circumstances where the .NET framework world will not release resources. This may be because the object references resources outside of the .NET framework, like the operating system, or internally references another unmanaged component, or that the resources accesses a component that uses COM, COM+ or DCOM.

Whatever the reason, if you are using an object that implements the IDisposable interface at a class level, then you too need to implement the IDisposable interface too.

public interface IDisposable
{
     public void Dispose();
}

This interface exposes a method called Dispose(). This alone will not help tidy up resources, as it is only an interface, so the developer must use it correctly in order to ensure the resources are released. The two steps are:

1. Always call Dispose() on any object that implements IDisposable as soon as you are finished using it. (This can be made easier with the using keyword)
2. Use the finalizer method to call Dispose(), so that if anyone has not closed your resources, your code will do it for them.

[edit] Dispose pattern

Often, what you want to clean up varies depending on whether your object is being finalized. For example, you would not want to clean up managed resources in a finalizer since the managed resources could have been reclaimed by the garbage collector already. The dispose pattern can help you implement resource management properly in this situation:

public class MyResource : IDisposable
{
    private IntPtr _someUnmanagedResource;
    private List<long> _someManagedResource = new List<long>();
 
    public MyResource()
    {
        _someUnmanagedResource = AllocateSomeMemory();
 
        for (long i = 0; i < 10000000; i++)
            _someManagedResource.Add(i);
        ...
    }
 
    // The finalizer will call the internal dispose method, telling it not to free managed resources.
    ~MyResource()
    {
        this.Dispose(false);
    }
 
    // The internal dispose method.
    private void Dispose(bool disposing)
    {
        if (disposing)
        {
            // Clean up managed resources
            _someManagedResource.Clear();
        }
 
        // Clean up unmanaged resources
        FreeSomeMemory(_someUnmanagedResource);
    }
 
    // The public dispose method will call the internal dispose method, telling it to free managed resources.
    public void Dispose()
    {
        this.Dispose(true);
        // Tell the garbage collector to not call the finalizer because we have already freed resources.
        GC.SuppressFinalize(this);
    }
}

[edit] Applications

If you are coming to C# from Visual Basic Classic you will have seen code like this:

  Public Function Read(ByRef FileName) As String
 
    Dim oFSO As FileSystemObject
    Set oFSO = New FileSystemObject
 
    Dim oFile As TextStream
    Set oFile = oFSO.OpenTextFile(FileName, ForReading, False)
    Read = oFile.ReadLine
 
  End Function

Note that neither oFSO nor oFile are explicitly disposed of. In Visual Basic Classic this is not necessary because both objects are declared locally. This means that the reference count goes to zero as soon as the function ends which results in calls to the Terminate event handlers of both objects. Those event handlers close the file and release the associated resources.

In C# this doesn't happen because the objects are not reference counted. The finalizers will not be called until the garbage collector decides to dispose of the objects. If the program uses very little memory this could be a long time.

This causes a problem because the file is held open which might prevent other processes from accessing it.

In many languages the solution is to explicitly close the file and dispose of the objects and many C# programmers do just that. However, there is a better way: use the using statement:

 public read(string fileName)
 {
    using (TextReader textReader = new StreamReader(filename))
    {
       return textReader.ReadLine();
    }
 }

Behind the scenes the compiler turns the using statement into try..finally and produces this intermediate language (IL) code:

 .method public hidebysig static string  Read(string FileName) cil managed
 {
   // Code size       39 (0x27)
   .maxstack  5
   .locals init (class [mscorlib]System.IO.TextReader V_0,
            string V_1)
   IL_0000:  ldarg.0
   IL_0001:  newobj     instance void [mscorlib]System.IO.StreamReader::.ctor(string)
   IL_0006:  stloc.0
   .try
   {
     IL_0007:  ldloc.0
     IL_0008:  callvirt   instance string [mscorlib]System.IO.TextReader::ReadLine()
     IL_000d:  stloc.1
     IL_000e:  leave      IL_0025
     IL_0013:  leave      IL_0025
   }  // end .try
   finally
   {
     IL_0018:  ldloc.0
     IL_0019:  brfalse    IL_0024
     IL_001e:  ldloc.0
     IL_001f:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
     IL_0024:  endfinally
   }  // end handler
   IL_0025:  ldloc.1
   IL_0026:  ret
 } // end of method Using::Read

Notice that the body of the Read function has been split into three parts: initialisation, try, and finally. The finally block includes code that was never explicitly specified in the original C# source code, namely a call to the destructor of the Streamreader instance.

See Understanding the 'using' statement in C# By TiNgZ aBrAhAm.

See the following sections for more applications of this technique.

[edit] Resource Acquisition Is Initialisation

The application of the using statement in the introduction is an example of an idiom called Resource Acquisition Is Initialisation (RAII).

RAII is a natural technique in languages like Visual Basic Classic and C++ that have deterministic finalization but usually requires extra work to include in programs written in garbage collected languages like C# and VB.NET. The using statement makes it just as easy. Of course you could write the try..finally code out explicitly and in some cases that will still be necessary. For a thorough discussion of the RAII technique see HackCraft: The RAII Programming Idiom. Wikipedia has a brief note on the subject as well: Resource Acquisition Is Initialization.

Work in progress: add C# versions showing incorrect and correct methods with and without using. Add notes on RAII, memoization and cacheing (see OOP wikibook). Design Patterns are common building blocks designed to solve everyday software issues. Some basic terms and example of such patterns include what we see in everyday life. Key patterns are the singleton pattern, the factory pattern, and chain of responsibility patterns.

[edit] Factory Pattern

The factory pattern is a method call that uses abstract classes and its implementations, to give the developer the most appropriate class for the job.

Lets create a couple of classes first to demonstrate how this can be used. Here we take the example of a bank system.

public abstract class Transaction
{
     private string _sourceAccount;
 
     //May not be needed in most cases, but may on transfers, closures and corrections.
     private string _destinationAccount;
 
     private decimal _amount;
     public decimal Amount { get { return _amount; } }
 
     private DateTime _transactionDate;
     private DateTime _effectiveDate;
 
     public Transaction(string source, string destination, decimal amount)
     {
          _sourceAccount = source;
          _destinationAccount = destination;
          _amount = amount;
          _transactionDate = DateTime.Now;
     }
 
     public Transaction(string source, string destination, decimal amount, DateTime effectiveDate) : this(source, destination, amount)
     {
          _effectiveDate = effectiveDate;
     }
 
     protected decimal AdjustBalance(string accountNumber, decimal amount)
     {
          decimal newBalance = decimal.MinValue;
 
          using(Mainframe.ICOMInterface mf = new Mainframe.COMInterfaceClass())
          {
               string dateFormat = DateTime.Now.ToString("yyyyMMdd HH:mm:ss");
               mf.Credit(dateFormat, accountNumber, amount);
               newBalance = mf.GetBalance( DateTime.Now.AddSeconds(1), accountNumber);
          }
 
          return newBalance;
     }
 
     public abstract bool Complete();
}

This Transaction class is incomplete, as there are many types of transactions:

• Opening
• Credits
• Withdrawals
• Transfers
• Penalty
• Correction
• Closure

For this example, we will take credit and withdrawal portions, and create classes for them.

public class Credit : Transaction
{
     //Implementations hidden for simplicity
 
     public override bool Complete()
     {
          this.AdjustBalance( _sourceAccount, amount);
     }
}
 
public class Withdrawal : Transaction
{
     //Implementations hidden for simplicity
 
     public override bool Complete()
     {
          this.AdjustBalance( _sourceAccount, -amount);
     }
}

The problem is that these classes do much of the same thing, so it would be helpful if we could just give it the values, and it will work out what class type we require. Therefore, we could come up with some ways to distinguish between the different types of transactions:

• Positive values indicate a credit.
• Negative values indicate a withdrawal.
• Having 2 account numbers and a positive value would indicate a transfer.
• Having 2 account numbers and a negative value would indicate a closure.
• etc

So, let us write a new class with a static method that will do this logic for us, ending the name Factory:

public class TransactionFactory
{
     public static Transaction Create( string source, string destination, decimal amount )
     {
          if( string.IsNullOrEmpty(destination) )
          {
               if(amount >= 0)
                    return new Credit( source, null, amount);
               else
                    return new Withdrawal( source, null, amount);
          }
          else
          {
               //Other implementations here
          }
     }
}

Now, you can use this class to do all of the logic and processing, and be assured that the type you are returned is correct.

public class MyProgram
{
     static void Main()
     {
          decimal randomAmount = new Random().Next() * 1000000;
          Transaction t = TransactionFactory.Create("123456","",randomAmount);
          //t.Complete(); <-- This would carry out the requested transaction.
 
          Console.WriteLine("{0}: {1:C}",t.GetType().Name, t.Amount);
     }
}

[edit] Singleton

The singleton pattern instantiates only 1 object, and reuses this object for the entire lifetime of the process. This is useful if you wish the object to maintain state, or if it takes lots of resources to set the object up. Below is a basic implementation:

public class MySingletonExample
{
   private Hashtable sharedHt;
 
   public Hashtable Singleton
   {
      get 
      {
         if(sharedHt == null)
            sharedHt = new Hashtable();
 
         return sharedHt;
      }
 
      set { ; } //Not implemented as this would invalidate the pattern
   }
 
   //Class implementation here..
}

The Singleton property will expose the same instance to all callers. Upon the first call, the object is initialised and on subsequent calls this is used.

Examples of this pattern include:

• HttpApplication (Application object in ASP .NET)
• HttpServerUtility (Server object in ASP .NET)
• HttpCacheUtility (Cache object in ASP .NET)
• ConfigurationSettings (Generic settings reader)

[edit] The .NET Framework

.NET Framework is a common environment for building, deploying, and running Web Services, Web Applications, Windows Services and Windows Applications. The .NET Framework contains common class libraries - like ADO.NET, ASP.NET and Windows Forms - to provide advanced standard services that can be integrated into a variety of computer systems.

[edit] Introduction

C# is a language in itself. It can perform mathematical and logical operation, variable assignment and other expected traits of a programming language. This in itself is not flexible enough for more complex applications. At some stage, the developer will want to interact with the host system whether it be reading files or downloading content from the internet.

The .NET framework is a toolset developed for the Windows platform to allow the developer to interact with the host system or any external entity whether it be another process, or another computer. The .NET platform is a Windows platform specific implementation. Other operating systems have their own implementations due to the differences in the operating systems I/O management, security models and interfaces.

[edit] Background

• .NET was originally called NGWS(Next Generation Windows Services).
• .NET does not run IN any browser. It is a RUNTIME language (Common Language Runtime) like the Java runtime. Microsoft Silverlight does run in a browser.
• .NET is based on the newest Web standards.
• .NET is built on the following Internet standards
  • HTTP, the communication protocol between Internet Applications
  • XML, the format for exchanging data between Internet Applications
  • SOAP, the standard format for requesting Web Services
  • UDDI, the standard to search and discover Web Services

[edit] Console Programming

[edit] Output

The example program below shows a couple of ways to output text:

using System;
public class HelloWorld
{
   public static void Main()
   {
       Console.WriteLine("Hello World!");             // relies on "using System;"
       Console.Write("This is");
       Console.Write("... my first program!\n");
       System.Console.WriteLine("Goodbye World!");    // no "using" statement required
   }
}

The above code displays the following text:

Hello World!
This is... my first program!
Goodbye World!

That text is output using the System.Console class. The using statement at the top allows the compiler to find the Console class without specifying the System namespace each time it is used.

The middle lines use the Write() method, which does not automatically create a new line. To specify a new line, we can use the sequence backslash-n ( \n ). If for whatever reason we wanted to really show the \n character instead, we add a second backslash ( \\n ). The backslash is known as the escape character in C# because it is not treated as a normal character, but allows us to encode certain special characters (like a new line character).

[edit] Input

Input can be gathered in a similar method to outputing data using the Read() and ReadLine methods of that same System.Console class:

using System;
public class ExampleClass
{
   public static void Main()
   {
       Console.WriteLine("Greetings!  What is your name?");
       Console.Write("My name is: ");
       string name = Console.ReadLine();
       Console.WriteLine("Nice to meet you, " + name);
       Console.Read();
   }
}

The above program requests the user's name and displays it back. The final Console.Read() waits for the user to enter a key before exiting the program.

[edit] Error

The Error output is used to divert error specific messages to the console. To a novice user this may seem fairly pointless, as this achieves the same as Output (as above). If you decide to write an application that runs another application (for example a scheduler), you may wish to monitor the output of that program - more specifically, you may only wish to be notified only of the errors that occur. If you coded your program to write to the Console.Error stream whenever an error occurred, you can tell your scheduler program to monitor this stream, and feedback any information that is sent to it. Instead of the Console appearing with the Error messages, your program may wish to log these to a file.

You may wish to revisit this after studying Streams and after learning about the Process class.

[edit] Command line arguments

Command line arguments are values that are passed to a console program before execution. For example, the Windows command prompt includes a copy command that takes two command line arguments. The first argument is the original file and the second is the location or name for the new copy. Custom console applications can have arguments as well.

using System;
public class ExampleClass
{
   public static void Main(string[] args)
   {
       Console.WriteLine("First Name: " + args[0]);
       Console.WriteLine("Last Name: " + args[1]);
       Console.Read();
   }
}

If the program above code is compiled to a program called username.exe, it can be executed from the command line using two arguments, e.g. "Bill" and "Gates":

C:\>username.exe Bill Gates

Notice how the Main() method above has a string array parameter. The program assumes that there will be two arguments. That assumption makes the program unsafe. If it is run without the expected number of command line arguments, it will crash when it attempts to access the missing argument. To make the program more robust, we can check to see if the user entered all the required arguments.

using System;
public class Test
{
   public static void Main(string[] args)
   {
       if(args.Length >= 1)
           Console.WriteLine(args[0]);
       if(args.Length >= 2)
           Console.WriteLine(args[1]);
   }
}

Try running the program with only entering your first name or no name at all. The args.Length property returns the total number of arguments. If no arguments are given, it will return zero.

You are also able to group a single argument together by using the "" quote marks. This is particularly useful if you are expecting many parameters, but there is a requirement for including spaces (e.g. file locations, file names, full names etc)

using System;
 
class Test
{
   public static void Main(string[] args)
   {
      for(int index =0 ;index < args.Length; index++)
      {
         Console.WriteLine((index+1) + ": " + args[index]);
      }
   }
}

C:\> Test.exe Separate words "grouped together"
1: Separate
2: words
3: grouped together

[edit] Formatted output

This section is a stub. You can help Wikibooks by expanding it.

Console.Write() and Console.WriteLine() allow you to output a text string, but also allows writing a string with variable substitution.

These two functions normally have a string as the first parameter. When additional objects are added, either as parameters or as an array, the function will scan the string to substitute objects in place of tokens.

For example:

{
  int i=10;
  Console.WriteLine("i = {0}", i);
}

The {0} is identified by braces, and refers to the parameter index which needs to be substituted. You may also find a format specifier wihtin the braces, which is preceded by a colon and the specifier in question (e.g. {0:G}).

The System.Windows.Forms namespace allows us to create Windows applications easily. The Form class is a particularly important part of that namespace because the form is the key graphical building block of Windows applications. It provides the visual frame that holds buttons, menus, icons, and title bars together. Integrated development environments (IDEs) like Visual C# and SharpDevelop can help create graphical applications, but it is important to know how to do so manually:

using System.Windows.Forms;
public class ExampleForm : Form    // inherits from System.Windows.Forms.Form
{
   public static void Main()
   {
       ExampleForm wikibooksForm = new ExampleForm();
       wikibooksForm.Text = "I Love Wikibooks";// specify title of the form
       wikibooksForm.Width = 400;              // width of the window in pixels
       wikibooksForm.Height = 300;             // height in pixels
       Application.Run(wikibooksForm);         // display the form
   }
}

The example above creates a simple Window with the text "I Love Wikibooks" in the title bar. Custom form classes like the example above inherit from the System.Windows.Forms.Form class. Setting any of the properties Text, Width, and Height is optional. Your program will compile and run successfully if you comment these lines out, but they allow us to add extra control to our form.

[edit] Events

This section is a stub. You can help Wikibooks by expanding it.

Event is a action being taken by the program when,say, for example, a Button is clicked. Event handler is a object that handles the event, that is, what action should be taken when an event is fired.

[edit] Controls

This section is a stub. You can help Wikibooks by expanding it.

[edit] Lists

A list is a dynamic array which resizes itself as needed if more data is inserted than it can hold at the time of insertion. Items can be inserted at any index, deleted at any index and accessed at any index. The C# non-generic list class is the ArrayList, while the generic one is List<T>.

Many of the List class' methods and properties are demonstrated in the following example:

using System;
using System.Collections;
using System.Collections.Generic;
 
namespace csharp_generic_list
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("List<T> demo");
            // creating an instance which accepts strings
            List<string> foods = new List<string>();
 
            // adding some items one by one with Add()
            foods.Add("bread");
            foods.Add("butter");
            foods.Add("chocolate");
 
            // adding a simple string array with AddRange()
            string[] subList1 = {"orange", "apple", "strawberry", "grapes", "kiwi", "banana"};
            foods.AddRange(subList1);
 
            // adding another List<string> with AddRange()
            List<string> anotherFoodList = new List<string>();
            anotherFoodList.Add("yoghurt");
            anotherFoodList.Add("tomato");
            anotherFoodList.Add("roast beef");
            anotherFoodList.Add("vanilla cake");
            foods.AddRange(anotherFoodList);
 
            // removing "orange" with Remove()
            foods.Remove("orange");
 
            // removing the 5th (index = 4) item ("strawberry") with RemoveAt()
            foods.RemoveAt(4);
 
            // removing a range (4-7: all fruits) with RemoveRange(int index, int count)
            foods.RemoveRange(3, 4);
 
            // sorting the list
            foods.Sort();
 
            // printing the sorted foods
            foreach (string item in foods)
            {
                Console.Write("| " + item + " ");
            }
            Console.WriteLine("|");
 
            // removing all items from foods
            foods.Clear();
 
            // printing the current item count in foods
            Console.WriteLine("The list now has {0} items.", foods.Count);
        }
    }
}

The terminal output is:

List<T> demo
| bread | butter | chocolate | roast beef | tomato | vanilla cake | yoghurt |
The list now has 0 items.

[edit] LinkedLists

Items in a linked list can be accessed directly only one after the other. Of course an item at any index can be accessed, but the list must iterate to the item from the first one, which is much slower than accessing items by index in an array or a list. There is no non-generic linked list in C#, while the generic one is LinkedList<T>.

[edit] Queues

A queue is a FIFO (first in - first out) collection. The item first pushed in the queue gets taken first with the pop function. Only the first item is accessible at any time, and items can only be put to the end. The non-generic queue class is called Queue, while the generic one is Queue<T>.

[edit] Stacks

A stack is a LIFO (last in - first out) collection. The item pushed in first will be the last to be taken by pop. Only the last item is accessible at any time, and items can only be put at the top. The non-generic stack class is Stack, while the generic one is Stack<T>.

[edit] Dictionaries

A dictionary is a collection of values with keys. The values can be very complex, yet searching the keys is still fast. The non-generic class is Hashtable, while the generic one is Dictionary<TKey, TValue>.

Threads are tasks which can run concurrently to other threads and can share data. When your program starts, it creates a thread for the entry point of your program, usually a Main function. So, you can think of a "program" as being made up of threads. The .NET Framework allows you to use threading in your programs to run code in parallel to each other. This is often done for two reasons:

1. If the thread running your graphical user interface performs time-consuming work, your program may appear to be unresponsive. Using threading, you can create a new thread to perform tasks and report its progress to the GUI thread.
2. On computers with more than one CPU or CPUs with more than one core, threads can maximize the use of computational resources, speeding up tasks.

[edit] The Thread class

The System.Threading.Thread class exposes basic functionality for using threads. To create a thread, you simply create an instance of the Thread class with a ThreadStart or ParameterizedThreadStart delegate pointing to the code the thread should start running. For example:

using System;
using System.Threading;
 
public static class Program
{
    private static void SecondThreadFunction()
    {
        while (true)
        {
            Console.WriteLine("Second thread says hello.");
            Thread.Sleep(1000); // pause execution of the current thread for 1 second (1000 ms)
        }
    }
 
    public static void Main()
    {
        Thread newThread = new Thread(new ThreadStart(SecondThreadFunction));
 
        newThread.Start();
 
        while (true)
        {
            Console.WriteLine("First thread says hello.");
            Thread.Sleep(500); // pause execution of the current thread for half a second (500 ms)
        }
    }
}

You should see the following output:

First thread says hello.
Second thread says hello.
First thread says hello.
First thread says hello.
Second thread says hello.
...

Notice that the while keyword is needed because as soon as the function returns, the thread exits, or terminates.

[edit] ParameterizedThreadStart

The void ParameterizedThreadStart(object obj) delegate allows you to pass a parameter to the new thread:

using System;
using System.Threading;
 
public static class Program
{
    private static void SecondThreadFunction(object param)
    {
        while (true)
        {
            Console.WriteLine("Second thread says " + param.ToString() + ".");
            Thread.Sleep(500); // pause execution of the current thread for half a second (500 ms)
        }
    }
 
    public static void Main()
    {
        Thread newThread = new Thread(new ParameterizedThreadStart(SecondThreadFunction));
 
        newThread.Start(1234); // here you pass a parameter to the new thread
 
        while (true)
        {
            Console.WriteLine("First thread says hello.");
            Thread.Sleep(1000); // pause execution of the current thread for a second (1000 ms)
        }
    }
}

The output is:

First thread says hello.
Second thread says 1234.
Second thread says 1234.
First thread says hello.
...

[edit] Sharing Data

Although we could use ParameterizedThreadStart to pass parameter(s) to threads, it is not typesafe and is clumsy to use. We could exploit anonymous delegates to share data between threads, however:

using System;
using System.Threading;
 
public static class Program
{
    public static void Main()
    {
        int number = 1;
        Thread newThread = new Thread(new ThreadStart(delegate
        {
            while (true)
            {
                number++;
                Console.WriteLine("Second thread says " + number.ToString() + ".");
                Thread.Sleep(1000);
            }
        }));
 
        newThread.Start();
 
        while (true)
        {
            number++;
            Console.WriteLine("First thread says " + number.ToString() + ".");
            Thread.Sleep(1000);
        }
    }
}

Notice how the body of the anonymous delegate can access the local variable number.

[edit] Synchronization

In the previous example, you may have noticed that often, if not all of the time, you will get the following output:

First thread says 2.
Second thread says 3.
Second thread says 5.
First thread says 4.
Second thread says 7.
First thread says 7.

One would expect that at least, the numbers would be printed in ascending order! This problem arises because of the fact that the two pieces of code are running at the same time. For example, it printed 3, 5, then 4. Let us examine what may have occurred:

1. After "First thread says 2", the first thread incremented number, making it 3, and printed it.
2. The second thread then incremented number, making it 4.
3. Just before the second thread got a chance to print number, the first thread incremented number, making it 5, and printed it.
4. The second thread then printed what number was before the first thread incremented it, that is, 4. Note that this may have occurred due to console output buffering.

The solution to this problem is to synchronize the two threads, making sure their code doesn't interleave like it did. C# supports this through the lock keyword. We can put blocks of code under this keyword:

using System;
using System.Threading;
 
public static class Program
{
    public static void Main()
    {
        int number = 1;
        object numberLock = new object();
        Thread newThread = new Thread(new ThreadStart(delegate
        {
            while (true)
            {
                lock (numberLock)
                {
                    number++;
                    Console.WriteLine("Second thread says " + number.ToString() + ".");
                }
 
                Thread.Sleep(1000);
            }
        }));
 
        newThread.Start();
 
        while (true)
        {
            lock (numberLock)
            {
                number++;
                Console.WriteLine("First thread says " + number.ToString() + ".");
            }
 
            Thread.Sleep(1000);
        }
    }
}

The variable numberLock is needed because the lock keyword only operates on reference types, not value types. This time, you will get the correct output:

First thread says 2.
Second thread says 3.
Second thread says 4.
First thread says 5.
Second thread says 6.
...

The lock keyword operates by trying to gain an exclusive lock on the object passed to it (numberLock). It will only release the lock when the code block has finished execution (that is, after the }). If an object is already locked when another thread tries to gain a lock on the same object, the thread will block (suspend execution) until the lock is released, and then lock the object. This way, sections of code can be prevented from interleaving.

[edit] Thread.Join()

The Join method of the Thread class allows a thread to wait for another thread, optionally specifying a timeout:

using System;
using System.Threading;
 
public static class Program
{
    public static void Main()
    {
        Thread newThread = new Thread(new ThreadStart(delegate
        {
            Console.WriteLine("Second thread reporting.");
            Thread.Sleep(5000);
            Console.WriteLine("Second thread done sleeping.");
        }));
 
        newThread.Start();
        Console.WriteLine("Just started second thread.");
        newThread.Join(1000);
        Console.WriteLine("First thread waited for 1 second.");
        newThread.Join();
        Console.WriteLine("First thread finished waiting for second thread. Press any key.");
        Console.ReadKey();
    }
}

The output is:

Just started second thread.
Second thread reporting.
First thread waited for 1 second.
Second thread done sleeping.
First thread finished waiting for second thread. Press any key.

The .NET Framework currently supports calling unmanaged functions and using unmanaged data, a process called marshalling. This is often done to use Windows API functions and data structures, but can also be used with custom libraries.

[edit] GetSystemTimes

A simple example to start with is the Windows API function GetSystemTimes. It is declared as:

BOOL WINAPI GetSystemTimes(
  __out_opt  LPFILETIME lpIdleTime,
  __out_opt  LPFILETIME lpKernelTime,
  __out_opt  LPFILETIME lpUserTime
);

LPFILETIME is a pointer to a FILETIME structure, which is simply a 64-bit integer. Since C# supports 64-bit numbers through the long type, we can use that. We can then import and use the function as follows:

using System;
using System.Runtime.InteropServices;
 
public class Program
{
    [DllImport("kernel32.dll")]
    static extern bool GetSystemTimes(out long idleTime, out long kernelTime, out long userTime);
 
    public static void Main()
    {
        long idleTime, kernelTime, userTime;
 
        GetSystemTimes(out idleTime, out kernelTime, out userTime);
        Console.WriteLine("Your CPU(s) have been idle for: " + (new TimeSpan(idleTime)).ToString());
        Console.ReadKey();
    }
}