静态构造函数(Static Constructor)(It performs well in Singleton)

今天无意间看到了静态构造函数的代码，也许是没怎么见过，觉得新鲜，就顺手写了以下代码，或许我见过，或许我忘记了……呵呵，发布上来自己作个纪念……

发现用来做Singleton模式倒是不错，呵呵……

using System;

namespace CA_StaticConstructor
{
    class StaticClass
    {
        public static StaticClass Instance { get; set; }
        public string Name { get; set; }
        static StaticClass()
        {
            if (Instance == null)
            {
                Instance = new StaticClass();
            } 
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            StaticClass sc1 = StaticClass.Instance;
            sc1.Name = "sc1 Instance";
            Console.WriteLine(sc1.Name);

            StaticClass sc2 = StaticClass.Instance;
            //Without set Name property to Instance
            Console.WriteLine(sc2.Name);
        }
    }
}

//Result:
//sc1 Instance
//sc1 Instance

以上代码可以简化为以下形式

using System;

namespace CA_StaticConstructor
{
    class StaticClass
    {
        public static StaticClass Instance { get; set; }
        public string Name { get; set; }
        static StaticClass()
        {
            Instance = new StaticClass();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            StaticClass sc1 = StaticClass.Instance;
            sc1.Name = "sc1 Instance";
            Console.WriteLine(sc1.Name);

            StaticClass sc2 = StaticClass.Instance;
            //Without set Name property to Instance
            Console.WriteLine(sc2.Name);
        }
    }
}
//Result:
//sc1 Instance
//sc1 Instance

注意上文中static StaticClass() 部分的代码，在传统的singleton中总是需要判断如第一段代码的构造，但是由于静态构造器在第一次调用的时候将会invoke，因此在后续的调用中将会保证实例的存在，即使加了也不为过，可以避免如下形式的代码破坏：

        static void Main(string[] args)
        {
            StaticClass sc1 = StaticClass.Instance;
            sc1.Name = "sc1 Instance";
            Console.WriteLine(sc1.Name);

            StaticClass.Instance = null;

            StaticClass sc2 = StaticClass.Instance;
            //Without set Name property to Instance
            Console.WriteLine(sc2.Name);
        }

当然按照常规我们最好不要也不应该有set方法，这样我们就可以保证我们的Instance始终是第一次调用的实例了……

using System;

namespace CA_StaticConstructor
{
    class StaticClass
    {
        private static readonly StaticClass instance;
        public static StaticClass Instance
        {
            get
            {
                return instance;
            }
        }
        public string Name { get; set; }
        static StaticClass()
        {
            instance = new StaticClass();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            StaticClass sc1 = StaticClass.Instance;
            sc1.Name = "sc1 Instance";
            Console.WriteLine(sc1.Name);

            //StaticClass.Instance = null;

            StaticClass sc2 = StaticClass.Instance;
            //Without set Name property to Instance
            Console.WriteLine(sc2.Name);
        }
    }
}
//Result:
//sc1 Instance
//sc1 Instance