设计模式

单例模式：
懒汉模式和饿汉模式。

考虑多线程安全问题，以及双重锁定。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Singleton
    {
        private static Singleton singleton;
        private static readonly object syncRoot = new object();

        private Singleton()
        {
 
        }

        public static Singleton GetInstance()
        {
            // multiple threading issue
            // double-check locking
            if (singleton == null)
            {
                lock (syncRoot)
                {
                    if (singleton == null)
                    {
                        singleton = new Singleton();
                    }
                }
            }

            return singleton;
        }
    }

    sealed class SingletonB
    {
        private static readonly SingletonB instance = new SingletonB();
        private SingletonB()
        {
 
        }

        public static SingletonB GetInstance()
        {
            return instance;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Singleton s1 = Singleton.GetInstance();
            Singleton s2 = Singleton.GetInstance();

            if(s1 == s2)
            {
                Console.WriteLine("Equals.");
            }

            Console.ReadLine();
        }
    }
}