创建型模式类（单件模式）

   1: namespace DoFactory.GangOfFour.Singleton.RealWorld { // MainApp test application 

   2:     class MainApp { static void Main() 

   3:     { 

   4:         LoadBalancer b1 = LoadBalancer.GetLoadBalancer(); 

   5:         LoadBalancer b2 = LoadBalancer.GetLoadBalancer(); 

   6:         LoadBalancer b3 = LoadBalancer.GetLoadBalancer(); 

   7:         LoadBalancer b4 = LoadBalancer.GetLoadBalancer(); 

   8:         // Same instance? 

   9:         if (b1 == b2 && b2 == b3 && b3 == b4) 

  10:         { 

  11:             Console.WriteLine("Same instance/n"); 

  12:         } 

  13:         // All are the same instance -- use b1 arbitrarily 

  14:         // Load balance 15 server requests 

  15:         for (int i = 0; i < 15; i++) 

  16:         { 

  17:             Console.WriteLine(b1.Server); 

  18:         } 

  19:         // Wait for user 

  20:         Console.Read(); 

  21:     } 

  22: } 

  23: //"Singleton" 

  24: class LoadBalancer 

  25: { 

  26:     private static 

  27:     LoadBalancer instance; 

  28:     private ArrayList servers = new ArrayList(); 

  29:     private Random random = new Random(); 

  30:     // Lock synchronization object private static 

  31:     object syncLock = new object();

  32:     // Constructor (protected) 

  33:     protected LoadBalancer() 

  34:     { 

  35:         // List of available servers 

  36:         servers.Add("ServerI"); 

  37:         servers.Add("ServerII");     

  38:         servers.Add("ServerIII"); 

  39:         servers.Add("ServerIV"); 

  40:         servers.Add("ServerV");

  41:     } 

  42:     public static LoadBalancer GetLoadBalancer() 

  43:     { 

  44:         //Support multithreaded applications through 

  45:         // 'Double checked locking' pattern which (once 

  46:         // the instance exists) avoids locking each 

  47:         // time the method is invoked 

  48:         if (instance == null) 

  49:         { 

  50:             lock (syncLock) 

  51:             { 

  52:                 if (instance == null) 

  53:                 { 

  54:                     instance = new LoadBalancer(); 

  55:                 } 

  56:             } 

  57:         } 

  58:         return instance; 

  59:     } 

  60:     // Simple, but effective random load balancer 

  61:     public string Server 

  62:     { 

  63:         get { int r = random.Next(servers.Count); return servers[r].ToString(); 

  64:             } 

  65:     } 

  66: } 

  67: }