public class Stock
{
   private ArrayList observers = new ArrayList();
   private Random rand = new Random();

   public Stock()
   {
     Timer timer = new Timer(3000);
     //Register an ElapsedEventHandler which will periodically triggere GeneratePrice
     timer.Elapsed +=new ElapsedEventHandler(GeneratePrice);
     timer.Start();
   }

   public void register(Observer anObserver)
   {
      observers.Add(anObserver);
   }

   private void GeneratePrice(object sender, ElapsedEventArgs e)
   {
      double price = rand.NextDouble() * 100;
      // We will notify the observers now
      foreach(Object o in observers)
      {
         Observer anObserver = o as Observer;
         anObserver.update(price);
      }
   }
}							

public interface Observer
{
   void update(Object data);
}

public class StockMonitor : Observer
{
   public void update(Object data)
   {
      Console.WriteLine("Price is " + data);
   }
}

Stock s = new Stock();
StockMonitor m = new StockMonitor();
s.register(m);