class1.cs

来自DoFactory的设计模式, 由于是本人根据已有代码创建,没有NETOptimized项

// Singleton pattern -- Real World example  

using System;
using System.Collections;
using System.Threading;

namespace DoFactory.GangOfFour.Singleton.RealWorld
{

    // MainApp test application 

    class MainApp
    {
        static void Main()
        {
            LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
            LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
            LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
            LoadBalancer b4 = LoadBalancer.GetLoadBalancer();

            // Same instance? 
            if (b1 == b2 && b2 == b3 && b3 == b4)
            {
                Console.WriteLine("Same instance\n");
            }

            // All are the same instance -- use b1 arbitrarily 
            // Load balance 15 server requests 
            for (int i = 0; i < 15; i++)
            {
                Console.WriteLine(b1.Server);
            }

            // Wait for user 
            Console.Read();
        }
    }

    // "Singleton" 

    class LoadBalancer
    {
        private static LoadBalancer instance;
        private ArrayList servers = new ArrayList();

        private Random random = new Random();

        // Lock synchronization object 
        private static object syncLock = new object();

        // Constructor (protected) 
        protected LoadBalancer()
        {
            // List of available servers 
            servers.Add("ServerI");
            servers.Add("ServerII");
            servers.Add("ServerIII");
            servers.Add("ServerIV");
            servers.Add("ServerV");
        }

        public static LoadBalancer GetLoadBalancer()
        {
            // Support multithreaded applications through 
            // 'Double checked locking' pattern which (once 
            // the instance exists) avoids locking each 
            // time the method is invoked 
            if (instance == null)
            {
                lock (syncLock)
                {
                    if (instance == null)
                    {
                        instance = new LoadBalancer();
                    }
                }
            }

            return instance;
        }

        // Simple, but effective random load balancer 

        public string Server
        {
            get
            {
                int r = random.Next(servers.Count);
                return servers[r].ToString();
            }
        }
    }
}