C#: my derived class cannot override base class's interface method implementation, why?

Question

I've got the code below, I use class "B" to inherit class "A" while I wish to implement F function from interface IMy. But compiler tells my I'm hiding interface method "F". So the running result is "A".

I expect this program to output "B". I don't wish to use implicit interface implementation as I wish to use normal polymorphism in main function.

How to correct my code? Thanks.

public interface IMy
{
    void F();
}

public class A : IMy
{
    public void F()
    {
        Console.WriteLine("A");
    }
}

public class B : A
{
    public void F()
    {
        Console.WriteLine("B");
    }
}
class Program
{
    static void Main(string[] args)
    {
        IMy my = new B();
        my.F();
    }
}

Answer 1

To override a method in C#, the method in base class needs to be explicitly marked as virtual . It doesn't matter if the method implements an interface method or not.

public class A : IMy
{
    public virtual void F()
    {
        Console.WriteLine("A");
    }
}

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

Answer 2

Your current code is equivalent to:

public class A : IMy
{
    public void F()
    {
        Console.WriteLine("A");
    }
}

public class B : A
{
    public new void F() // <--- Note 'new' here
    {
        Console.WriteLine("B");
    }
}

You're implicitly marking the method as new, which will generate a compiler warning unless you explicitly write it.

What you actually want is to override the method, so declare that:

public class A : IMy
{
    public virtual void F() // <--- Mark as virtual to allow subclasses to override
    {
        Console.WriteLine("A");
    }
}

public class B : A
{
    public override void F() // <-- Override the method rather than hiding it
    {
        Console.WriteLine("B");
    }
}