using System;

​

BaseClass a = new DerivedClass();

a.SayHello();

DerivedClass b = new DerivedClass();

b.SayHello();

public class BaseClass

{

    public void SayHello()

    {

        Console.WriteLine("Hello world!");

    }

}

​

public class DerivedClass : BaseClass

{

    public void SayHello()

    {

        Console.WriteLine("Hello Artem!");

    }

}