C# 单例模式

 

C# 单例模式

    一、多线程不安全方式实现

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public sealed class SingleInstance    {        private static SingleInstance instance;        private SingleInstance() { }        public static SingleInstance Instance        {            get            {                if (null == instance)                {                    instance = new SingleInstance();                }                return instance;            }        }    }

　　sealed表示SingleInstance不能被继承。其实构造函数私有化已经达到了这个效果，私有的构造函数不能被继承。为了可读性，可以加个sealed。

不安全的单例指的是在多线程环境下可能有多个线程同时进入if语句，创建了多次单例对象。

   二、安全的单例模式

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public sealed class SingleInstance   {       private static volatile SingleInstance instance;       private static readonly object obj = new object();       private SingleInstance() { }       public static SingleInstance Instance       {           get           {               if (null == instance)               {                   lock (obj)                   {                       if (null == instance)                       {                           instance = new SingleInstance();                       }                   }                 }               return instance;           }       }   }

　加锁保护，在多线程下可以确保实例值被创建一次。缺点是每次获取单例，都要进行判断，涉及到的锁和解锁比较耗资源。

三、只读属性式

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public sealed class SingleInstance    {        private static readonly SingleInstance instance = new SingleInstance();        private SingleInstance() { }        public static SingleInstance Instance        {            get            {                return instance;            }        }    }

　　 借助readonly属性，instance只被初始化一次，同样达到了单例的效果。在Main函数执行第一句话之前，instance其实已经被赋值了，并不是预期的 只有到访问Instance变量时才创建对象。

如下代码：

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class Program    {        static void Main(string[] args)        {            Console.WriteLine("Begin");            var temp = SingleInstance.instance; ;        }    }      public sealed class SingleInstance    {        public static readonly SingleInstance instance = new SingleInstance();        private SingleInstance()        {            Console.WriteLine("初始化初始化！");        }        public static SingleInstance Instance        {            get { return instance; }        }    }

　　输出：

在执行第一句代码之前，实例已经被初始化。

解决方法是在SingleInstance中加上静态构造函数。

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public sealed class SingleInstance    {        public static readonly SingleInstance instance = new SingleInstance();        static SingleInstance() { }        private SingleInstance()        {            Console.WriteLine("初始化初始化！");        }        public static SingleInstance Instance        {            get { return instance; }        }    }

　　在运行输出：

   

 

四、使用Lazy

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public sealed class SingleInstance    {        private static readonly Lazy instance = new Lazy(() => new SingleInstance());        private SingleInstance(){}        public static SingleInstance Instance        {            get            {                return instance.Value;            }        }    }

　　Lazy默认是线程安全的。MSDN描述如下：

 Will the lazily initialized object be accessed from more than one thread? If so, the Lazy object might create it on any thread. You can use one of the simple constructors whose default behavior is to create a thread-safe Lazy object, so that only one instance of the lazily instantiated object is created no matter how many threads try to access it. To create a Lazy object that is not thread safe, you must use a constructor that enables you to specify no thread safety.

   

五、泛型单例

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class Program {     static void Main(string[] args)     {         Console.WriteLine("Begin");         mySingle.Instance.age = 500;         Console.WriteLine(mySingle.Instance.age);     } }   public abstract class SingleInstance {     private static readonly Lazy _instance = new Lazy(() =>         {             var ctors = typeof(T).GetConstructors(BindingFlags.Instance| BindingFlags.NonPublic| BindingFlags.Public);             if (ctors.Count() != 1)                 throw new InvalidOperationException(String.Format("Type {0} must have exactly one constructor.", typeof(T)));             var ctor = ctors.SingleOrDefault(c => c.GetParameters().Count() == 0 && c.IsPrivate);             if (ctor == null)                 throw new InvalidOperationException(String.Format("The constructor for {0} must be private and take no parameters.", typeof(T)));             return (T)ctor.Invoke(null);         });     public static T Instance     {       get{ return _instance.Value;}     } }   public class mySingle : SingleInstance {     private mySingle() { }     public int age; }