单例模式

    //1、线程不安全
    //public sealed class Singleton
    //{
    //    private static Singleton instance; //定义私有的静态全局变量保存改类的唯一实例
    //    private Singleton() { }
    //    public static Singleton GetInstance()
    //    {
    //        //如果该类没有被实例化，才去创建改实例
    //        return instance ?? (instance=new Singleton());
    //    }
    //}


    //2、线程安全
    //public sealed class Singleton
    //{
    //    private static Singleton instance = null;
    //    //创建静态只读的进程辅助对象
    //    private static readonly object _lock = new object();
    //    Singleton() { }
    //    public static Singleton GetInstance()
    //    {
    //        lock (_lock)
    //        {
    //            return instance ?? (instance = new Singleton());
    //        }
    //    }
    //}



    //3、线程安全 双重锁定
    //public sealed class Singleton
    //{
    //    private static Singleton instance;
    //    private static object _lock = new object();
    //    private Singleton() { }
    //    public static Singleton GetInstance()
    //    {
    //        if (instance == null)//实例为空时，才加锁创建
    //        {
    //            lock (_lock)//保证只有一个线程可以访问改语句块
    //            {
    //                    instance = instance ?? (new Singleton());
    //            }
    //        }
    //        return instance;
    //    }
    //}




    //4、线程安全 静态初始化
    //public sealed class Singleton
    //{
    //    private static Singleton instance = new Singleton();
    //    private Singleton() { }
    //    public static Singleton GetInstance()
    //    {
    //        return instance;
    //    }
    //}


    //5、延迟初始化，内部类
    //public sealed class Singleton
    //{
    //    private Singleton() { }
    //    public static Singleton GetInstance()
    //    {
    //        return Nested.instance;
    //    }
    //    //初始化工作放到Nested类中的静态成员来完成
    //    private class Nested
    //    {
    //        static Nested() { }
    //        internal static readonly Singleton instance = new Singleton();
    //    }
    //}


    //6、延迟初始化，线程安全
    public sealed class Singleton
    {
        private static readonly Lazy<Singleton> lazy = new Lazy<Singleton>(() => new Singleton());
        public static Singleton GetInstance()
        {
            return lazy.Value;
        }
        private Singleton() { }   
    }