// Mediator pattern -- Real World example using System;using System.Collections;namespace DoFactory.GangOfFour.Mediator.RealWorld{  // MainApp test application  class MainApp {  static void Main()  {   // Create chatroom    Chatroom chatroom = new Chatroom();   // Create participants and register them    Participant George = new Beatle("George");   Participant Paul = new Beatle("Paul");   Participant Ringo = new Beatle("Ringo");   Participant John = new Beatle("John") ;   Participant Yoko = new NonBeatle("Yoko");   chatroom.Register(George);   chatroom.Register(Paul);   chatroom.Register(Ringo);   chatroom.Register(John);   chatroom.Register(Yoko);   // Chatting participants    Yoko.Send ("John", "Hi John!");   Paul.Send ("Ringo", "All you need is love");   Ringo.Send("George", "My sweet Lord");   Paul.Send ("John", "Can't buy me love");   John.Send ("Yoko", "My sweet love") ;   // Wait for user    Console.Read();  } } // "Mediator"  abstract class AbstractChatroom {  public abstract void Register(Participant participant);  public abstract void Send(   string from, string to, string message); } // "ConcreteMediator"  class Chatroom : AbstractChatroom {  private Hashtable participants = new Hashtable();  public override void Register(Participant participant)  {   if (participants[participant.Name] == null)   {    participants[participant.Name] = participant;   }   participant.Chatroom = this;  }  public override void Send(   string from, string to, string message)  {   Participant pto = (Participant)participants[to];   if (pto != null)   {    pto.Receive(from, message);   }  } } // "AbstractColleague"  class Participant {  private Chatroom chatroom;  private string name;  // Constructor   public Participant(string name)  {   this.name = name;  }  // Properties   public string Name  {   get{ return name; }  }  public Chatroom Chatroom  {   set{ chatroom = value; }   get{ return chatroom; }  }  public void Send(string to, string message)  {   chatroom.Send(name, to, message);  }  public virtual void Receive(   string from, string message)  {   Console.WriteLine("{0} to {1}: '{2}'",    from, Name, message);  } } //" ConcreteColleague1"  class Beatle : Participant {  // Constructor   public Beatle(string name) : base(name)   {   }  public override void Receive(string from, string message)  {   Console.Write("To a Beatle: ");   base.Receive(from, message);  } } //" ConcreteColleague2"  class NonBeatle : Participant {  // Constructor   public NonBeatle(string name) : base(name)   {   }  public override void Receive(string from, string message)  {   Console.Write("To a non-Beatle: ");   base.Receive(from, message);  } }}