class Singleton

{

    private static Singleton instance=null;  //静态私有成员变量

    //私有构造函数

    private Singleton()

    {

    }

    //静态公有工厂方法，返回唯一实例

    public static Singleton GetInstance()

    {

        if(instance==null)

            instance=new Singleton();

        return instance;

    }

}

class EagerSingleton

{

private static EagerSingleton instance = new EagerSingleton();

private EagerSingleton() { }

public static EagerSingleton GetInstance()

{

return instance;

}

}

class LazySingleton

{

private static LazySingleton instance = null;

//程序运行时创建一个静态只读的辅助对象

private static readonly object syncRoot = new object();

private LazySingleton() { }

public static LazySingleton GetInstance()

{

      //第一重判断，先判断实例是否存在，不存在再加锁处理

if (instance == null)

{

    //加锁的程序在某一时刻只允许一个线程访问

lock(syncRoot)

{

    //第二重判断

    if(instance==null)

    {

        instance = new LazySingleton();  //创建单例实例

}

}

}

return instance;

}

}