单例模式

描述：单例模式要求一个类仅有一个实例，并且提供了一个全局的访问点。
方式一（推荐）

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/// <summary> /// 单例模式，静态初始化 /// sealed:密封类，不可被继承，防止子类被实例化而不能保证只有一个实例。 /// private Singleton():用private修改构造函数，防止类在外部被实例。 /// static readonly:表示只能在声明时赋值，或是在静态构造中赋值。 /// </summary> public sealed class Singleton {     private static readonly Singleton instance = new Singleton();     private Singleton() { }     public static Singleton getInstance()     {         return instance;     } }

　　方式二（不推荐）加锁费资源

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/// <summary>     /// 单例模式，加锁双层判空，延迟加载     /// </summary>     public sealed class Singleton2     {         private static Singleton2 instance;         private Singleton2() { }         private static readonly object obj = new object();         public static Singleton2 getInstance()         {             if (instance == null)             {                 lock (obj)                 {                     if (instance == null)                     {                         instance = new Singleton2();                     }                 }             }             return instance;         }     }

 方式三（推荐）

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/// <summary>     /// 单例模式，静态初始化，延迟加载     /// </summary>     public sealed class Singleton3     {         public Singleton3() { }         public static Singleton3 Instance         {             get             {                 return Delay.DelayInstance;             }         }         public sealed class Delay         {             private static readonly Singleton3 delayInstance = new Singleton3();             private Delay() { }             public static Singleton3 DelayInstance             {                 get                 {                     return delayInstance;                 }             }         }     }

 应用场景：Windows任务管理器、windows回收站、网站的计数器、日志应用、WebConfig对象的读取、数据库连接池、多线程的线程池、HttpApplication 
参考：
http://www.cnblogs.com/Terrylee/category/36516.html
http://www.cnblogs.com/Terrylee/archive/2005/12/09/293509.html