Question

I have some code like this:

How should I implement the operator == so that it will be called when the variables are of interface IMyClass?

public class MyClass : IMyClass
{
    public static bool operator ==(MyClass a, MyClass b)
    {
        if (ReferenceEquals(a, b))
            return true;

        if ((Object)a == null || (Object)b == null)
            return false;

        return false;
    }

    public static bool operator !=(MyClass a, MyClass b)
    {
        return !(a == b);
    }
}

class Program
{
    static void Main(string[] args)
    {
        IMyClass m1 = new MyClass();
        IMyClass m2 = new MyClass();

        MyClass m3 = new MyClass();
        MyClass m4 = new MyClass();

        Console.WriteLine(m1 == m2); // does not go into custom == function. why not?
        Console.WriteLine(m3 == m4); // DOES go into custom == function
    } 
}

Answer 1

The key is that you’re not overriding an operator – you’re overloading it.

There’s no operator defined for

operator ==(IMyClass x, IMyClass y)

so the compiler has nothing it could call. It can’t call

operator ==(MyClass x, MyClass y)

as it doesn’t know that m1 and m2 will actually refer to instance of MyClass.

As far as I know there’s no way of implementing an operator to be used for interfaces – after all, multiple implementations could all provide their own one, just for one point of possible ambiguity.

Personally I’m somewhat wary of trying to talk about equality over non-sealed types to start with – equality and inheritance don’t mix terribly nicely. That goes doubly for interfaces, of course 🙂 You might be best implementing an appropriate IEqualityComparer<IMyClass> and using that instead.