装箱与拆箱C#（一）

局部变量值类型直接包含了它们的数据，而接口和Object包含的是它们的引用，当从一个值类型转换为它所实现的接口或基类object时，即装箱

装箱（从值类型转换到引用类型）步骤：

1.首先在堆中分配好内存。用于存放值类型的数据以及少许额外开销（包括方法表指针、SyncBlock-Index）

2.之后进行内存复制，栈上的值类型数据复制到堆上分配好的位置

3.更新对象或接口的引用，指向堆的位置

拆箱则与之相反。CIL指令unbox只对堆上的数据进行解引用，不包括从堆复制到栈的动作。但C#中多数会紧接在拆箱后发生一次复制动作。

下面介绍通过CIL查看，在一个特定代码片段中统计box/unbox指令的数量

             C#代码                                                                 CIL代码

static void Main(string[] args)                                    .method private hidebysig

                                                                                             static void Main()     cil managed

        {                                                                                   {      .entrypoint

                                                                                                   //code size    21(0*15)

                                                                                                  .maxstack   1

            int  number;                                                                 .locals  init      (   [0]  int 32  number,

            object  thing;                                                                                           [1]  object   thing)

            number = 42;                                                                IL_0000:nop

                                                                                           
    IL_0001:ldc.i4.s            42

                                                                                               IL_0003:stloc.0

            //Boxing                                                                       IL_0004:ldloc.0                                         

            thing = number;                                                            IL_0005:box                    [mscorlib]System.Int32

                                                                                              IL_000a:stloc.1

             //Unboxing                                                                  IL_000b:ldloc.1

            number = (int)thing;                                                       IL_000c:unbox.any          [mscorlib]System.Int32

                                                                                               IL_0011:stloc.0

                                                                                               IL_0012:br.s                                 IL_0014

            return;                                                                          IL_0014:ret

        }                                                                                   }   //  end of method Program::Main

装箱进行不频繁，性能问题不大。但装箱易忽略！！！