using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        // Configure Observer pattern

        ConcreteSubject s = new ConcreteSubject();

        s.Attach(new ConcreteObserver(s, "X"));

        s.Attach(new ConcreteObserver(s, "Y"));

        s.Attach(new ConcreteObserver(s, "Z"));

        // Change subject and notify observers

        s.SubjectState = "ABC";

        s.Notify();

    }

}

​

/// <summary>

/// The 'Subject' abstract class

/// </summary>

abstract class Subject

{

    private List<Observer> _observers = new List<Observer>();

    public void Attach(Observer observer)

    {

        _observers.Add(observer);

    }

​

    public void Detach(Observer observer)

    {

        _observers.Remove(observer);

    }

​

    public void Notify()

    {

        foreach (Observer o in _observers)

        {

            o.Update();

        }

    }

}

​

/// <summary>

/// The 'ConcreteSubject' class

/// </summary>

class ConcreteSubject : Subject

{

    private string _subjectState;

    // Gets or sets subject state

    public string SubjectState

    {

        get

        {

            return _subjectState;

        }

​

        set

        {

            _subjectState = value;

        }

    }

}

​

/// <summary>

/// The 'Observer' abstract class

/// </summary>

abstract class Observer

{

    public abstract void Update();

}

​

/// <summary>

/// The 'ConcreteObserver' class

/// </summary>

class ConcreteObserver : Observer

{

    private string _name;

    private string _observerState;

    private ConcreteSubject _subject;

    // Constructor

    public ConcreteObserver(ConcreteSubject subject, string name)

    {

        this._subject = subject;

        this._name = name;

    }

​

    public override void Update()

    {

        _observerState = _subject.SubjectState;

        Console.WriteLine("Observer {0}'s new state is {1}", _name, _observerState);

    }

​

    // Gets or sets subject

    public ConcreteSubject Subject

    {

        get

        {

            return _subject;

        }

​

        set

        {

            _subject = value;

        }

    }

}