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