using System;

using System.Collections.Generic;

namespace DoFactory.GangOfFour.Singleton.NETOptimized

{

  /// <summary>

​

  /// MainApp startup class for .NET optimized 

​

  /// Singleton Design Pattern.

​

  /// </summary>

​

  class MainApp

​

  {

    /// <summary>

​

    /// Entry point into console application.

​

    /// </summary>

​

    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();

    }

  }

  /// <summary>

​

  /// The 'Singleton' class

​

  /// </summary>

​

  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<Server> _servers;

    private Random _random = new Random();

    // Note: constructor is 'private'

​

    private LoadBalancer()

    {

      // Load list of available servers

​

      _servers = new List<Server> 

        { 

         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];

      }

    }

  }

  /// <summary>

​

  /// Represents a server machine

​

  /// </summary>

​

  class Server

​

  {

    // Gets or sets server name

​

    public string Name { get; set; }

    // Gets or sets server IP address

​

    public string IP { get; set; }

  }

}