How do I specify a C# return value with an ambiguous generic parameter?

Question

In C#, suppose I have two interfaces: Foo<in P, out C> and Bar<C> . Also suppose I have two implementations of Bar<int> : BarOne and BarTwo .

I want to add a method Baz() on Bar<C> that returns something like:

public interface Bar<C> {
  // other methods, some of which use C as a type
  // then:
  Foo<..., C> Baz ();
}

public class BarOne : Bar<int> {
  // other methods...
  private class FooImplOne : Foo<string, int> {
    // stuff here
  }
  Foo<string, int> Baz() {
    return new FooImplOne();
  }
}

public class BarTwo : Bar<int> {
  // other methods...
  private class FooImplTwo : Foo<long, int> {
    // stuff here
  }
  Foo<long, int> Baz() {
    return new FooImplTwo();
  }
}

But I can't say anything about the first parameter of Foo in Baz 's return in the definition of the Bar interface. In java I'd use Foo<?, C> as the return type in the Bar definition - what do I do in C#?

I found a 2008 stackoverflow answer that tells me "In C# you can't do that, and need to define a base interface for Foo<P, C> that only has the generic parameter C and return that base type instead".

Is that still the case in modern, 2016 C#?

Answer 1

In C# you would also make the Method a generic method with a different type parameter local to that method

eg

public interface Bar<C> {
  Foo<T, C> Baz<T>();
}

See MSDN: Generic Methods

Answer 2

how about

public interface Bar<T1,T2> {
  Foo<T1, T2> Baz ();
}

ie pass both Foo type params in

Answer 3

In Java, <?> is shorthand for <? extends Object> <? extends Object> . This means that the code that is trying to use it only knows that it is an object. The equivalent in C# would just be using <object> , so your interface would just be

Foo<object, C> Baz ();