Observer Design Pattern in C#

Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

This structural code demonstrates the Observer pattern in which registered objects are notified of and updated with a state change.

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; }
    }
  }