创建型设计模式之单例模式

 1 using System;
 2 
 3     class Singleton 
 4     {
 5         private static Singleton _instance;
 6         
 7         public static Singleton Instance()
 8         {
 9             if (_instance == null)
10                 _instance = new Singleton();
11             return _instance;
12         }
13         protected Singleton(){}
14 
15         // Just to prove only a single instance exists
16         private int x = 0;
17         public void SetX(int newVal) {x = newVal;}
18         public int GetX(){return x;}        
19     }
20 
21     /// <summary>
22     ///    Summary description for Client.
23     /// </summary>
24     public class Client
25     {
26         public static int Main(string[] args)
27         {
28             int val;
29             // can‘t call new, because constructor is protected
30             Singleton FirstSingleton = Singleton.Instance(); 
31             Singleton SecondSingleton = Singleton.Instance();
32 
33             // Now we have two variables, but both should refer to the same object
34             // Let‘s prove this, by setting a value using one variable, and 
35             // (hopefully!) retrieving the same value using the second variable
36             FirstSingleton.SetX(4);
37             Console.WriteLine("Using first variable for singleton, set x to 4");        
38 
39             val = SecondSingleton.GetX();
40             Console.WriteLine("Using second variable for singleton, value retrieved = {0}", val);        
41             return 0;
42         }
43     }