Question

I have a class B that inherits from class A, which in turn inherits from interface I. This interface exposes a method M which, of course, is implemented by A, but I would like to override it in B. Moreover, I would like to call A.M from B.M. How do I do that?

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EDIT: The answers made me feel kind of stupid, especially since I know what virtual means and, in fact, I have tried it:

class A
{
    virtual void I.M()           // fails

I never even considered not implementing the interface explicitly.

Thank you all.

Answer 1

Well, either you need to make the method virtual in A, or you need to reimplement the interface in B, which gets messy. Here’s the simpler version:

using System;

public interface IFoo
{
    void M();
}

public class A : IFoo
{
    public virtual void M()
    {
        Console.WriteLine("A.M");
    }
}

public class B : A
{
    public override void M()
    {
        base.M();
        Console.WriteLine("B.M");
    }
}

class Test
{
    static void Main()
    {
        IFoo foo = new B();
        foo.M();
    }
}

… and here’s the version which reimplements IFoo, hiding A.M() instead of overriding it:

using System;

public interface IFoo
{
    void M();
}

public class A : IFoo
{
    public void M()
    {
        Console.WriteLine("A.M");
    }
}

public class B : A, IFoo
{
    public new void M()
    {
        base.M();
        Console.WriteLine("B.M");
    }
}

class Test
{
    static void Main()
    {
        IFoo foo = new B();
        foo.M();
    }
}

Note that if you then had:

A a = (A) foo;
a.M();

it would only call A.M(), not B.M().