单例模式的三种写法和优缺点

 1 using System;
 2 using System.Collections.Generic;
 3 
 4 namespace ConsoleApp1
 5 {
 6     class Singleton
 7     {
 8         private static Singleton _singleton = null;
 9         private static object singleton_Lock = new object();  //对象锁
10 
11         /// <summary>
12         /// 标准的单例模式
13         /// </summary>
14         /// <returns></returns>
15         public static Singleton CreateInstance()
16         {
17             if (_singleton == null)//保证对象初始化化之后的所有线程，不需要等待锁
18             {
19                 lock (singleton_Lock)//保证只有一个线程进去判断
20                 {
21                     if (_singleton == null)//保证对象为空才创建
22                     {
23                         _singleton = new Singleton();
24                     }
25                 }
26             }
27             return _singleton;
28         }
29    }//Class_end
30 }

 1 using System;
 2 using System.Collections.Generic;
 3 
 4 namespace ConsoleApp1
 5 {
 6     class Singleton
 7     {
 8         private static Singleton _Singleton = null;
 9 
10         /// <summary>
11         /// 静态构造函数：由CLR保证，在第一次调用之前调用，只调用一次
12         /// </summary>
13         static Singleton()
14         {
15             _Singleton = new Singleton();
16         }
17 
18         public static Singleton CreateInstance()
19         {
20             return _Singleton;
21         }
22       }//Class_end
23 }

 1 using System;
 2 using System.Collections.Generic;
 3 
 4 namespace ConsoleApp1
 5 {
 6     class Singleton
 7     {
 8         //静态变量会在类型第一次使用的时候初始化，而且只初始化一次
 9         private static Singleton _singleton = new Singleton();
10         public static Singleton CreateInstance()
11         {
12             return _singleton;
13         }
14       }//class_end
15 }