Can I Enforce a Subclass to Implement an Interface?

Question

I have a base class, say named B. I have two derived classes, D1 and D2.

Looks like:

public abstract class B
{
    public abstract void DoSomething();
}

public class D1 : B
{
    public override void DoSomething() { ... }
}

public class D2 : B
{
    public override void DoSomething() { ... }
}

Now, I created a new interface IDeepCopyable<T> which has one method Clone():

public interface IDeepCopyable<T>
{
    T Clone();
}

I want to force each subclass too implement this interface with the base class (i.e. IDeepCopyable<B>.

If I try to leave B's declaration as-is but just inherit from ('implement' is a more accurate term?) IDeepCopyable<T>, such as:

public abstract class B : IDeepCopyable<B>

Then to implement it in the derived classes (either implicitly or explicitly), the compiler gives me an error message "B does not implement interface member IDeepCopyable<B>.Clone()".

I can, of course, create an abstract method whose name is identical to the interface's method's name, but I find that ugly. Why to redeclare the method?

Can I, in any way, leave that as wanted?

I see that VS2019 has an option "Implement interface abstractly" so I think that the answer is no, but A) I want to be sure. B) If so, is there a design concept behind this behavior, or is it just a bug in C# design?

Thanks in advance.

Answer 1

I can, of course, create an abstract method whose name is identical to the interface's method's name, but I find that ugly.

That's the way to go. Your base class implements the interface. Hence it has to fulfil it. It can do this by implementing the method concretely (which is not desired here) or the base class can force its inheritors to do so by declaring the method abstractly.

You say that's ugly, well I think it's just explicit. Anyway, there's no way around this ;-)