using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

}

​

public interface IObserver

{

    void callback();

};

​

​

public class OldObserver: IObserver

{

    public void callback()

    {

        Console.WriteLine("Call back in Old way"); 

    }

};

​

​

public class NewObserver: IObserver

{

    public void callback()

    {

       Console.WriteLine("Call back in New way"); 

    }

};

​

​

​

public interface ISubject

{

    void addObserver(IObserver observer); 

    void removeObserver(IObserver observer);

    void notifyObserver(IObserver observer);

};

​

public class Subject: ISubject

{

    public void addObserver(IObserver observer)

    {

    }

    public void removeObserver(IObserver observer)

    {

    }

    public void notifyObserver(IObserver observer)

    {

    }

}