readonly changing the behavior of a struct

Question

I'm trying to understand some basic concepts:

class Program
{
   private static readonly MyStruct m = new MyStruct();
   static void Main(string[] args)
    {
       //new MutableSample().RunSample();

       Console.WriteLine(m.ChangeInternal());
       Console.WriteLine(m.ChangeInternal());
       Console.WriteLine(m.ChangeInternal());
       Console.Read();
    }
}

public struct MyStruct
{
    private int x;
    public int ChangeInternal()
    {
        this.x = this.x + 1;
        return this.x;
    }
}

When I run this code it gives me 1, 1, 1, but when I remove the "readonly" it says 1, 2, 3.

Can someone explain to me this?

Answer 1

Section 7.5.4 of the C# specs states:

[...] if the field is readonly and the reference occurs outside an instance constructor of the class in which the field is declared, then the result is a value, namely the value of the field I in the object referenced by E

So when the field is readonly you're mutating a copy (since it's impossible to mutate a value, only a variable). When it isn't you're mutating the field itself.

This is described in more detail in this blog post by Eric Lippert. To quote its ending:

This is yet another reason why mutable value types are evil. Try to always make value types immutable.