让一个类只有一个对象，全局唯一

非多线程模式，实现方法：

方法1：

1     public class SingletonTest2     {3         public static readonly SingletonTest Instance = new SingletonTest();4         private SingletonTest() { }5     }

方法2：

1     public class SingletonTest2     {3         public static readonly SingletonTest Instance;4         static SingletonTest()5         {6             Instance = new SingletonTest();7         }8         private SingletonTest() { }9     }

 多线程模式，实现方法：

1     public class SingletonTest 2     { 3         private static volatile SingletonTest instance = null; 4         private static object lockobj = new object(); 5         private SingletonTest() { } 6         public static SingletonTest Instance 7         { 8             get 9             {10                 if (instance == null)11                 {12                     lock (lockobj)13                     {14                         if (instance == null)15                         {16                             instance = new SingletonTest();17                         }18                     }19                 }20                 return instance;21             }22         }23     }