C#单例模式源码

// Singleton

// Intent: "Ensure a class only has one instance, and provide a global
// point of access to it". 

// For further information, read "Design Patterns", p127, Gamma et al.,
// Addison-Wesley, ISBN:0-201-63361-2

/* Notes:
 * If it makes sense to have only a single instance of a class (a so-called
 * singleton), then it makes sense to enforce this (to elimintate potential 
 * errors, etc). 
 * 
 * A class based on the singleton design pattern protects its constructor, 
 * so that only the class itself (e.g. in a static method) may instantiate itself. 
 * It exposes an Instance method which allows client code to retrieve the 
 * current instance, and if it does not exist to instantiate it.  
 */
 
namespace Singleton_DesignPattern
{
    using System;

class Singleton 
{
private static Singleton _instance;

public static Singleton Instance()
{
if (_instance == null)