ict.ken.be

// Singleton pattern -- .NET optimized

using System;
using System.Collections.Generic;
 
namespace DoFactory.GangOfFour.Singleton.NETOptimized
{
  /// 

  /// MainApp startup class for .NET optimized
  /// Singleton Design Pattern.
  /// 
  class MainApp
  {
    /// 

    /// Entry point into console application.
    /// 
    static void Main()
    {
      LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b4 = LoadBalancer.GetLoadBalancer();
 
      // Confirm these are the same instance
      if (b1 == b2 && b2 == b3 && b3 == b4)
      {
        Console.WriteLine("Same instance\n");
      }
 
      // Next, load balance 15 requests for a server
      LoadBalancer balancer = LoadBalancer.GetLoadBalancer();
      for (int i = 0; i < 15; i++)
      {
        string serverName = balancer.NextServer.Name;
        Console.WriteLine("Dispatch request to: " + serverName);
      }
 
      // Wait for user
      Console.ReadKey();
    }
  }
 
  /// 

  /// The 'Singleton' class
  /// 
  sealed class LoadBalancer
  {
    // Static members are 'eagerly initialized', that is,
    // immediately when class is loaded for the first time.
    // .NET guarantees thread safety for static initialization
    private static readonly LoadBalancer _instance =
      new LoadBalancer();
 
    // Type-safe generic list of servers
    private List _servers;
    private Random _random = new Random();
 
    // Note: constructor is 'private'
    private LoadBalancer()
    {
      // Load list of available servers
      _servers = new List
        {
         new Server{ Name = "ServerI", IP = "120.14.220.18" },
         new Server{ Name = "ServerII", IP = "120.14.220.19" },
         new Server{ Name = "ServerIII", IP = "120.14.220.20" },
         new Server{ Name = "ServerIV", IP = "120.14.220.21" },
         new Server{ Name = "ServerV", IP = "120.14.220.22" },
        };
    }
 
    public static LoadBalancer GetLoadBalancer()
    {
      return _instance;
    }
 
    // Simple, but effective load balancer
    public Server NextServer
    {
      get
      {
        int r = _random.Next(_servers.Count);
        return _servers[r];
      }
    }
  }
 
  /// 

  /// Represents a server machine
  /// 
  class Server
  {
    // Gets or sets server name
    public string Name { get; set; }
 
    // Gets or sets server IP address
    public string IP { get; set; }
  }
}

Output:
Same instance

ServerIV
ServerIV
ServerIII
ServerV
ServerII
ServerV
ServerII
ServerII
ServerI
ServerIV
ServerIV
ServerII
ServerI
ServerV
ServerIV