Question

Is there any way to specify the allowed subclasses for an interface?

Let’s say I have this classes:

public interface IDoable // specify that only A or it's subclasses can implement IDoable
{
    void Do();
}

public class A : IDoable
{
    public void Do(){};
    public void Foo(){};
}

public class B : IDoable // implementing IDoable on something other than A or its sublcasses doesn't make any sense
{
    // public void Do();
}

So later I will be able to do this:

IDoable d = new A();
d.Do();
d.Foo(); // I'd like to be able to do this.

Does C# support this kind of feature?

Answer 1

You can cast d to A in order to be able to call Foo on it:

((A)d).Foo();

Note that if d is not A, this will throw an exception.

Other options are to test the type of d using is or as operators.

This of course goes against the point of using an interface to begin with.