单例模式的优化-双重锁定

三种单例模式写法：

    public class Singleton
    {
        private static Singleton instance;
        private static readonly object syncRoot = new object();
        private Singleton() { }
        public static Singleton GetInstance()
        {
            if (instance == null)//Double-Check Locking 双重锁定，比直接lock性能更好，最完美的模式（懒汉式单例）
            {
                lock (syncRoot)
                {
                    if (instance == null)
                    {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }
    }
    public class Singleton1
    {
        private static Singleton1 instance;
        private static readonly object syncRoot = new object();
        private Singleton1() { }
        public static Singleton1 GetInstance()
        {
            lock (syncRoot)//每次调用GetInstance都需要lock，影响性能（懒汉式单例）
            {
                if (instance == null)
                {
                    instance = new Singleton1();
                }
            }
            return instance;
        }
    }
    public class Singleton2
    {
        private static Singleton2 instance=new Singleton2();//静态初始化方式（饿汉式单例），类一加载就实例化对象，一般这样用也没问题，缺点：提前占用系统资源
        private Singleton2() { }
        public static Singleton2 GetInstance()
        {
            return instance;
        }
    }

双重锁定性能优化分析：