How to override a method inherited from a base class, which in turn implemented it from an interface?

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 AM from BM . How do I do that?

---

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 AM() 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 AM() , not BM() .

Answer 2

When you implement method M in class A, implement it as "virtual", then when you want to override it in class B use the "override" keyword, which will allow you to do just that.

interface I {
    void M();
}

class A : I {
    virtual void M() {

    }
}

class B : A {
    override void M() {
        //Do stuff;
        base.M();
    }

}

Answer 3

You can use the New keyword to hide the base M as well, heres a short linqpad program

The diffrence between virtual and new

http://blogs.msdn.com/b/csharpfaq/archive/2004/03/12/what-s-the-difference-between-code-override-code-and-code-new-code.aspx

void Main()
{
    var x = new B();
    x.M();
}

public interface I
{
    void M();
}
public class A : I
{
    public void M()
    {
        "A.M".Dump();
    }
}
public class B : A
{
    public new void M()
    {
        "B.M".Dump();
        base.M();
    }
}

Results:

B.M
A.M

Answer 4

Make M virtual in A

interface ISome
{
   void M();
}

class B : ISome
{
   public virtual M()
   {
   }
}

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