public abstract class Base

{

    public virtual void Foo()

    {

        Console.WriteLine("In Base");

    }

}

​

public class A : Base

{

    public void Foo()

    {

        Console.WriteLine("In A");

    }

}

​

public class B : Base

{

    public override void Foo()

    {

        Console.WriteLine("In B");

    }

}

​

Base a = new A();

Base b = new B();

​

a.Foo();

b.Foo();