C# and the readonly keyword

I came across something interesting today and thought I’d share this small bit of information to the world. While this made me scratch my head for a second or two it really does make sense. In C#, and I assume VB.NET as well, we have the keyword readonly which allows one to initialize a read only variable during declaration or within a constructor. I recently had the situation where I needed to use a readonly but couldn’t figure out the best area to place it, either in my parent class or in each child class created. Being one who believes less code is better I leaned towards just throwing it in my parent class. It turns out though, that a read only variable can ONLY be initialized during its declaration or in the class the read only variable resides in and children classes WILL NOT be allowed to initialize the variable.

Here is a quick and dirty code example of what I’m talking about…

  public class ClassA
  {
    protected readonly String stringA;
 
    public ClassA() {}
  }
 
  public class ClassB : ClassA
  {
    public ClassB(String value)
    {
      stringA = value; // ERROR: A readonly field cannot be assigned to (except in a constructor or a variable initializer)
    }
  }