Not sure if I interpreted the problem correctly, but in C# composing interfaces and using generic extension methods with type predicates:

interface IFooAble { ... }

interface IBarAble : IFooAble { ... }

static class FooProcessor {

    static T ProcessFoo<T>(T fooInst) 

        where T: IFooAble { ... }

}

This gives us something that as long as the object implements IFooAble, we can use this extension for free. I.E: IBarAble can use ProcessFoo just fine. This works really well for writing fluent interfaces as demoed above.