Question

Put default(T) in an interface. Explicitly implement the interface. The result does not compile

public interface IWhatever<T>
{
   List<T> Foo(T BarObject = default(T));
}

public class ConcreteWhatever: IWhatever<ConcreteWhatever>
{
    List<ConcreteWhatever> Foo(ConcreteWhatever BarObject = default(T)) {}
}

I fully expect default(ConcreteWhatever). What I get is default(T) which results in a compilation error.

I just go in and replace default(T) with null and things are fine. But this is hideous. Why is this happening?

Answer 1

You don’t have a T in this case, because ConcreteWherever isn’t a generic type.

If you want default(ConcreteWhatever) then that’s the code you should write.

Are you just complaining about the code auto-generated by Visual Studio? If so, that’s a reasonable complaint, but it would be worth being explicit about it… (Note that you’re not using explicit interface implementation here – otherwise it would be declared as IWhatever<ConcreteWhatever>.Foo. You don’t really have properly implicit implementation either, as otherwise it should be public…)

EDIT: I’ve just tried the same thing myself, and seen the same result, except the method is made public. Looks like it’s just a fault with Visual Studio – I suggest you create a Connect request for it. It’s a relatively rare situation though, I suspect – creating a generic interface which specifies an optional parameter which uses the default value of a type parameter as the value…