using System;

public class Program

{

    public static void Main()

    {

        ClassA a = new ClassB();

        Console.Write(a.DoSomething());

    }

}

​

public class ClassA

{

    public string DoSomething()

    {

        return "Blah from A";

    }

}

​

public class ClassB : ClassA

{

    public string DoSomething()

    {

        return "Blah from B";

    }

}

​

​