Thread safe Singleton pattern

关于Singleton模式的几个误区
网上有很多Singleton模式的样板, 多数是错误的, 下面的文档提出了很好的样板.
http://msdn.microsoft.com/en-us/library/ms998558.aspx
http://www.devhood.com/tutorials/tutorial_details.aspx?tutorial_id=486
http://www.yoda.arachsys.com/csharp/singleton.html
http://codebetter.com/blogs/steve.hebert/archive/2005/12/16/135697.aspx
另外特别说明的是:
1. Singleton class的Instance属性或者公开方法GetInstance()不应带参数, 也就是说无论在什么情况下获取Singleton类的对象, 都始终是那一个, 不会有不同的实例.
2. Singleton类不能是Static类, 因为Static类不能包含任何实例方法(包括实例constructor). 但Singleton类一定要包含一个static变量(比如_Instance, 其类型为Singleton)来存储Singleton类的实例, 那为什么必须是static变量呢? 因为我们在使用Singleton类的时候, 不是通过constructor类创建一个Singleton对象, 而是需要通过调用一个static方法(比如GetInstance())来创建一个实例, 而static方法只能使用static变量, 不能使用任何非static变量, 这就要求存放Singleton实例的内部变量必须是static变量.
3. Singleton类必须要使用lock, 否则就不是线程安全的, 除非_Instance这个static变量, 同时被声明为readonly.
4. 一般情况下, 不要Singleton模式, 不必使用Double check+Lock这样的代码, 因为写法复杂, 而且容易会造成线程不安全. 尤其是在多核机器上. 可以直接使用Single Check+Lock写法, 简单而又安全.

Bad Code	Good Code
// Bad code! Do not use! Not Thread safe, public sealed class Singleton { static Singleton instance=null; Singleton() { } public static Singleton Instance { get { if (instance==null) { instance = new Singleton(); } return instance; } } }	//Thread safe, 不是Lazy load, 特点:实现简单 //But performance suffers as a lock is acquired every time the instance is requested public sealed class Singleton { static Singleton instance = null; static readonly object padlock = new object(); Singleton() { } public static Singleton Instance { get { lock (padlock) { if (instance == null) { instance = new Singleton(); } return instance; } } } }
// Try to double Check, But still is not thread safe! Do not use! //Thread safe issue see: //http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html public sealed class Singleton { static Singleton instance=null; static readonly object padlock = new object(); Singleton() { } public static Singleton Instance { get { if (instance==null) { lock (padlock) { if (instance==null) { instance = new Singleton(); } } } return instance; } } }	//Thread safe,虽然不是Lazy load. 它使用了static Initilization,特点:简单 public sealed class Singleton { private static readonly Singleton instance = new Singleton(); private Singleton(){} public static Singleton Instance { get { return instance; } } }
// double Check, Thread not safe //Thread safe issue see: //http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html public sealed class Singleton { static Singleton instance=null; static readonly object padlock = new object(); Singleton() { } public static Singleton Instance { get { if (instance==null) { lock (padlock) { if (instance==null) { //线程不安全,因为编译器会进行代码优化, //临时变量tempInstance会被舍弃,这样就编程了线程不安全代码了 Singleton tempInstance=new Singleton(); instance = tempInstance; } } } return instance; } } }	// double Check, Thread safe public sealed class Singleton { volatile static Singleton instance=null; static readonly object padlock = new object(); Singleton() { } public static Singleton Instance { get { if (instance==null) { lock (padlock) { //线程安全,因为instance被标记为volatile if (instance==null) { Instance=new Singleton(); } } } return instance; } } }

thread-safe, not quite as lazy, without using locks	thread-safe, fully lazy instantiation
public sealed class Singleton { static readonly Singleton instance=new Singleton(); // Explicit static constructor to tell C# compiler // not to mark type as beforefieldinit static Singleton() { } Singleton() { } public static Singleton Instance { get { return instance; } } }	public sealed class Singleton { Singleton() { } public static Singleton Instance { get { return Nested.instance; } } class Nested { // Explicit static constructor to tell C# compiler // not to mark type as beforefieldinit static Nested() { } internal static readonly Singleton instance = new Singleton(); } }