using System;

public class Program

{

    public static void Main()

    {

        var a = new A();

        var b = new B();

        a.Speak();

        b.Speak();

        a.Speak2();

        b.Speak2();

    }

}

​

public class A

{

    public virtual void Speak()

    {

        Console.WriteLine("A");

    }

    public void Speak2()

    {

        Speak();

    }

}

​

public class B : A

{

    public override void Speak()

    {

        Console.WriteLine("B");

    }

}