C# Programming/Keywords/lock

From Wikibooks, open books for an open world
Jump to: navigation, search


The lock keyword allows a section of code to exclusively use a resource, a feature useful in multi-threaded applications. If a lock to the specified object is already held when a piece of code tries to lock the object, the code's thread is blocked until the object is available.

using System;
using System.Threading;
 
class LockDemo
{
    private static int number = 0;
    private static object lockObject = new object();
 
    private static void DoSomething()
    {
        while (true)
        {
            lock (lockObject)
            {
                int originalNumber = number;
 
                number += 1;
                Thread.Sleep((new Random()).Next(1000)); // sleep for a random amount of time
                number += 1;
                Thread.Sleep((new Random()).Next(1000)); // sleep again
 
                Console.Write("Expecting number to be " + (originalNumber + 2).ToString());
                Console.WriteLine(", and it is: " + number.ToString());
                // without the lock statement, the above would produce unexpected results, 
                // since the other thread may have added 2 to the number while we were sleeping.
            }
        }
    }
 
    public static void Main()
    {
        Thread t = new Thread(new ThreadStart(DoSomething));
 
        t.Start();
        DoSomething(); // at this point, two instances of DoSomething are running at the same time.
    }
}

The parameter to the lock statement must be an object reference, not a value type:

class LockDemo2
{
    private int number;
    private object obj = new object();
 
    public void DoSomething()
    {
        lock (this) // ok
        {
            ...
        }
 
        lock (number) // not ok, number is not a reference
        {
            ...
        }
 
        lock (obj) // ok, obj is a reference
        {
            ...
        }
    }
}



C# Keywords
abstract as base bool break
byte case catch char checked
class const continue decimal default
delegate do double else enum
event explicit extern false finally
fixed float for foreach
goto if implicit in int
interface internal is lock long
namespace new null object operator
out override params private protected
public readonly ref return sbyte
sealed short sizeof stackalloc
static string struct switch this
throw true try typeof uint
ulong unchecked unsafe ushort using
var virtual void volatile while
Special C# Identifiers
add alias get global partial
remove set value where yield