MainApp

using System;
using System.Collections;

  class MainApp
  {
    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();

      // Wait for user 
      Console.Read();
    }
  }

  // "Subject" 
  abstract class Subject
  {
    private ArrayList observers = new ArrayList();

    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();
      }
    }
  }

  // "ConcreteSubject" 
  class ConcreteSubject : Subject
  {
    private string subjectState;

    // Property 
    public string SubjectState
    {
      get{ return subjectState; }
      set{ subjectState = value; }
    }
  }

  // "Observer" 
  abstract class Observer
  {
    public abstract void Update();
  }

  // "ConcreteObserver" 
  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);
    }

    // Property 
    public ConcreteSubject Subject
    {
      get { return subject; }
      set { subject = value; }
    }
  }