Observer

// Observer pattern -- Real World example

using System; using System.Collections; namespace DoFactory.GangOfFour.Observer.RealWorld {       // MainApp test application    class MainApp   {      static void Main()     {        // Create investors       Investor s = new Investor("Sorros");       Investor b = new Investor("Berkshire");        // Create IBM stock and attach investors       IBM ibm = new IBM("IBM", 120.00);       ibm.Attach(s);       ibm.Attach(b);        // Change price, which notifies investors       ibm.Price = 120.10;       ibm.Price = 121.00;       ibm.Price = 120.50;       ibm.Price = 120.75;        // Wait for user       Console.Read();     }   }    // "Subject"    abstract class Stock   {      protected string symbol;      protected double price;      private ArrayList investors = new ArrayList();      // Constructor      public Stock( string symbol, double price)     {        this.symbol = symbol;        this.price = price;     }      public void Attach(Investor investor)     {       investors.Add(investor);     }      public void Detach(Investor investor)     {       investors.Remove(investor);     }      public void Notify()     {        foreach (Investor investor in investors)       {         investor.Update( this);       }       Console.WriteLine("");     }      // Properties      public double Price     {        get{ return price; }        set       {         price = value;         Notify();       }     }      public string Symbol     {        get{ return symbol; }        set{ symbol = value; }     }   }    // "ConcreteSubject"    class IBM : Stock   {      // Constructor      public IBM( string symbol, double price)       : base(symbol, price)     {     }   }    // "Observer"    interface IInvestor   {      void Update(Stock stock);   }    // "ConcreteObserver"    class Investor : IInvestor   {      private string name;      private Stock stock;      // Constructor      public Investor( string name)     {        this.name = name;     }      public void Update(Stock stock)     {       Console.WriteLine("Notified {0} of {1}'s " +         "change to {2:C}", name, stock.Symbol, stock.Price);     }      // Property      public Stock Stock     {        get{ return stock; }        set{ stock = value; }     }   } }