readonly keyword is closely related to the const keyword at a glance, with the exception of allowing a variable with this modifier to be initialized in a constructor, along with being associated with a class instance (object) rather than the class itself.
The primary use for this keyword is to allow the variable to take on different values depending on which constructor was called, in case the class has many, while still ensuring the developer that it can never intentionally or unintentionally be changed in the code once the object has been created.
This is a sample usage, assumed to be in a class called
readonly string s;
s = "Hello!";
|Special C# Identifiers (Contextual Keywords)
|Contextual Keywords (Used in Queries)