代码：单件模式（Singleton Pattern）

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


namespace DesignPatterns.Singletons
{
    //一个类有且仅有一个实例，并且提供了一个全局的访问点

    /**//// <summary>
    /// 简单
    /// </summary>
    public sealed class Singleton01
    {
        static Singleton01 instance = null;
        private Singleton01()
        {
        }
        static Singleton01 Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new Singleton01();
                }
                return instance;
            }
        }
    }

    /**//// <summary>
    /// 线程安全，单锁:每次都锁，性能欠佳
    /// </summary>
    public sealed class Singleton02
    {
        static Singleton02 instance = null;
        static readonly object olock = new object();
        private Singleton02()
        {
        }
        static Singleton02 Instance
        {
            get
            {
                lock (olock)
                {
                    if (instance == null)
                    {
                        instance = new Singleton02();
                    }
                    return instance;
                }                
            }
        }
    }

    /**//// <summary>
    /// 线程安全，双锁:为空时锁
    /// </summary>
    public sealed class Singleton03
    {
        static Singleton03 instance = null;
        static readonly object olock = new object();
        private Singleton03()
        {
        }
        static Singleton03 Instance
        {
            get
            {
                if (instance == null)
                {
                    lock (olock)
                    {
                        if (instance == null)//解决了线程并发问题，同时避免在每个 Instance 属性方法的调用中都出现独占锁定
                        {
                            instance = new Singleton03();
                        }
                    }                        
                }
                return instance;
            }
        }
    }

    /**//// <summary>
    /// 静态初始化,实例化机制的控制权较少
    /// </summary>
    public sealed class Singleton04
    {
        static readonly Singleton04 instance = new Singleton04();

        private Singleton04()
        {
        }
        static Singleton04()
        {
        }

        static Singleton04 Instance
        {
            get
            {
                return instance;
            }
        }
    }
}