c# – 用户定义的值类型的装箱

发布时间：2020-12-16 02:04:19 所属栏目：百科来源：网络整理

导读：根据MSDN,如果定义了结构,该结构应该覆盖从对象类继承的所有方法.建议在调用任何继承的方法(如ToString)时避免不必要的装箱. 根据MSDN,为了确定是否以及何时发生装箱,可以在MSIL代码中找到IL指令“框”. 我写了下面的测试来看拳击. using System;namespace T

根据MSDN,如果定义了结构,该结构应该覆盖从对象类继承的所有方法.建议在调用任何继承的方法(如ToString)时避免不必要的装箱.

根据MSDN,为了确定是否以及何时发生装箱,可以在MSIL代码中找到IL指令“框”.

我写了下面的测试来看拳击.

using System;

namespace TestingBoxing
{
    public struct StructX
    {
        public int member1;
        public int member2;
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            StructX s1;

            s1.member1 = 2;
            s1.member2 = 5;

            string str = s1.ToString();

            Console.WriteLine(str);
        }
    }
}

但是,尽管在结构定义中未调用ToString,但在下面的MSIL代码中无法看到装箱指令.

.method public hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       37 (0x25)
  .maxstack  2
  .locals init ([0] valuetype TestingBoxing.StructX s1,[1] string str)
  IL_0000:  ldloca.s   s1
  IL_0002:  ldc.i4.2
  IL_0003:  stfld      int32 TestingBoxing.StructX::member1
  IL_0008:  ldloca.s   s1
  IL_000a:  ldc.i4.5
  IL_000b:  stfld      int32 TestingBoxing.StructX::member2
  IL_0010:  ldloca.s   s1
  IL_0012:  constrained. TestingBoxing.StructX
  IL_0018:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_001d:  stloc.1
  IL_001e:  ldloc.1
  IL_001f:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0024:  ret
} // end of method Program::Main

怎么解释？

参考文章：http://msdn.microsoft.com/en-us/library/ms973858.aspx#code-snippet-6

解决方法

这可以通过查看Constrained的作用来解释.

字段通常受到约束,以便以标准方式使用callvirt,而无需明确选择框.它执行以下操作：

If thisType is a reference type (as opposed to a value type) then ptr is dereferenced and passed as the ‘this’ pointer to the callvirt of method.

If thisType is a value type and thisType implements method then ptr is passed unmodified as the ‘this’ pointer to a call method instruction,for the implementation of method by thisType.

If thisType is a value type and thisType does not implement method then ptr is dereferenced,boxed,and passed as the ‘this’ pointer to the callvirt method instruction.

这意味着什么(如MSDN文章所述)：

This last case can occur only when method was defined on Object,ValueType,or Enum and not overridden by thisType. In this case,the boxing causes a copy of the original object to be made. However,because none of the methods of Object,and Enum modify the state of the object,this fact cannot be detected.

强调我的.基本上说如果拳击确实发生,则无法通过IL确定.

受约束的MSDN：http://msdn.microsoft.com/en-us/library/system.reflection.emit.opcodes.constrained%28v=vs.110%29.aspx

（编辑：李大同）

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