【设计模式】单例模式

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单例模式

单例模式是设计模式中比较简单的一种设计模式。
有许多种写法：

第一种不考虑线程安全

    /// <summary>    /// First version - not thread-safe    /// </summary>    public sealed class Singleton    {        /// <summary>        /// 静态        /// </summary>        static Singleton instance = null;        /// <summary>        /// 私有构造函数        /// </summary>        Singleton()        { }        /// <summary>        /// public property        /// </summary>        public static Singleton Instance        {            get            {                if (instance == null)                {                    instance = new Singleton();                }                return instance;            }        }    }

第二种线程安全

    /// <summary>    /// Second version - simple thread-safety    /// </summary>    public sealed class Singleton    {        static Singleton instance = null;        /// <summary>        ///         /// </summary>        static readonly object padlock = new object();        Singleton()        { }        public static Singleton Instance        {            get            {                lock (padlock)                {                    if (instance == null)                    {                        instance = new Singleton();                    }                    return instance;                }            }        }    }

第三种我没看懂

    /// <summary>    /// Third version - attempted thread-safety using double-check locking    /// </summary>    public sealed class Singleton    {        static Singleton instance = null;        /// <summary>        ///         /// </summary>        static readonly object padlock = new object();        Singleton()        { }        public static Singleton Instance        {            get            {                if (instance == null)                {                    lock (padlock)                    {                        if (instance == null)                        {                            instance = new Singleton();                        }                        return instance;                    }                }                return instance;            }        }    }

第四种比较偷懒的写法

    /// <summary>    /// Fourth version - not quite as lazy, but thread-safe without using locks    /// </summary>    public sealed class Singleton    {        static readonly Singleton instance = new Singleton();        /// <summary>        /// static constructor        /// </summary>        static Singleton()        { }        public static Singleton Instance        {            get { return instance; }        }    }

第五种比较偷懒的写法

    /// <summary>    /// Fifth version - fully lazy instantiation    /// </summary>    public sealed class Singleton    {        Singleton()        {        }        public static Singleton Instance        {            get            {                return Nested.instance;            }        }        class Nested        {            // Explicit static constructor to tell C# compiler            // not to mark type as beforefieldinit            static Nested()            {            }            internal static readonly Singleton instance = new Singleton();        }    }

参考文章

Depth in C About Singleton

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