单例模式（Singleton）：
保证一个类仅有一个实例，并提供一个访问它的全局访问点。
一、UML结构图
二、示例代码【重点】
1 using System;
2 using System.Collections.Generic;
3 using System.Linq;
4 using System.Text;
5
6 namespace 单例模式
7 {
8 class Program
9 {
10 static void Main(string[] args)
11 {
12 SingleModelOne.GetInstance().Show();
13 SingleModelTwo.GetInstance().Show();
14 SingleModelThree.GetInstance().Show();
15
16 Console.Read();
17 }
18 }
19
20 /// <summary>
21 /// 单线程单例模式(懒汉式单例)
22 /// </summary>
23 public class SingleModelOne
24 {
25 private static SingleModelOne m_SingleModeOne = null;
26
27 /// <summary>
28 /// 单例模式的构造函数一定要【私有化】
29 /// </summary>
30 private SingleModelOne() { }
31
32 public static SingleModelOne GetInstance()
33 {
34 if (m_SingleModeOne == null)
35 m_SingleModeOne = new SingleModelOne();
36
37 return m_SingleModeOne;
38 }
39 public void Show()
40 {
41 Console.WriteLine("1、我是懒汉式、单线程单例模式");
42 }
43 }
44
45 /// <summary>
46 /// 多线程单例模式(懒汉式单例)
47 /// </summary>
48 public class SingleModelTwo
49 {
50 private static SingleModelTwo m_SingleModeTwo = null;
51 private static object m_LockObj = new object();
52
53 /// <summary>
54 /// 单例模式的构造函数一定要【私有化】
55 /// </summary>
56 private SingleModelTwo() { }
57
58 public static SingleModelTwo GetInstance()
59 {
60 //双重加锁 一 ：是为了防止线程阻塞，若对象已创建，此时在lock内部判断，就会导致其它线程在lock外等待。
61 if (m_SingleModeTwo == null)
62 {
63 lock (m_LockObj)
64 {
65 if (m_SingleModeTwo == null)
66 m_SingleModeTwo = new SingleModelTwo();
67 }
68 }
69 return m_SingleModeTwo;
70 }
71 public void Show()
72 {
73 Console.WriteLine("2、我是懒汉式、多线程单例模式");
74 }
75 }
76
77 /// <summary>
78 /// 单线程单例模式(饿汉式单例)
79 /// 饿汉式：【sealed】防止类被派生，派生就可能会增加实例。
80 /// 【readonly】对象只读，交由OCL负责初始化变量
81 /// </summary>
82 public sealed class SingleModelThree
83 {
84 //首先将对象初始化完毕
85 private readonly static SingleModelThree m_SingleModelThree = new SingleModelThree();
86
87 /// <summary>
88 /// 单例模式的构造函数一定要【私有化】
89 /// </summary>
90 private SingleModelThree() { }
91
92 public static SingleModelThree GetInstance()
93 {
94 return m_SingleModelThree;
95 }
96 public void Show()
97 {
98 Console.WriteLine("3、我是饿汉式、单线程单例模式");
99 }
100 }
101
102 }