[C# Tips] 有趣的类型静态构造器
这是C#中一个有趣的现象,也许您从中可以窥见些许CLR在构造类型时的行为,以及JIT编译的触发式编译过程。
看下面一段代码:
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class Program
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{
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static void Main()
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{
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myValueType1 type1 = new myValueType1();
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Console.WriteLine(myValueType1.myInt);
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Console.WriteLine("**********************");
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myValueType2 type2 = new myValueType2();
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type2.myInt = 123;
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Console.WriteLine(type2.myInt);
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Console.WriteLine("**********************");
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myValueType3 type3 = new myValueType3();
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}
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}
15![](/Images/OutliningIndicators/None.gif)
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struct myValueType1
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{
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static myValueType1()
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{
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Console.WriteLine("Hello from myValueType1");
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// myInt = 111;
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}
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public static Int32 myInt;
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}
25![](/Images/OutliningIndicators/None.gif)
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struct myValueType2
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{
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static myValueType2()
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{
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Console.WriteLine("Hello from myValueType2");
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}
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public Int32 myInt;
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}
34![](/Images/OutliningIndicators/None.gif)
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struct myValueType3
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{
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static myValueType3()
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{
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Console.WriteLine("Hello from myValueType3");
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myInt = 333;
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}
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public static Int32 myInt;
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}
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这里定义了三个结构:myValueType1,myValueType2,myValueType3。三个结构均带静态构造器,在构造器中都有一句用来输出的的代码。在myValueType1和myValueType3的静态。然后我们在main函数里面分别new 了相应的三个实例。您可以先想想输出的结果应该是怎样的。
事实上您会得到如下的结果:
我们看到虽然三个结构中都有静态构造器,却只有第一个结构的被执行了。事实上,这个有趣的现象也是CLR对性能的考虑,除非类型确实被访问到了,否则永远不会调用到它的类型构造器,这个过程是JIT的。
当执行到第六行代码时,CLR尝试要去myValueType1查找静态字段myInt的值。这个时候,myValueType1才是真正被访问到了。静态构造器被执行,得到相应的输出。
而myValueType2中myInt是个实例成员,访问它的值只关系到实例type2实例。与类型本身没有任何关系,CLR不会执行类型myValueType2的静态构造器。
myValueType3跟myValueType11几乎是一样的,myInt是静态成员,但是在main函数中,myValueType3还是没有被真正访问到,只是利用它构造出了一个虚拟的对象结构,这种对象结构里面所有字段都被赋予一个0值或者null值
所以第二行输出为零
这些性质与JIT编译器都是分不开的。