c# - 无法将 ref enum 转换为 ref dynamic？

Question

给定一个函数声明

dynamic DoSomething(dynamic inputObject)

我可以用枚举调用它作为inputObject:

MyEnum myEnum;
myEnum = DoSomething(myEnum);

但出于某种原因，如果函数将 inputObject 声明为类型 ref dynamic 而不是 dynamic:

dynamic DoSomething(ref dynamic inputObject)

由于转换无效，以下操作无效:

MyEnum myEnum;
DoSomething(ref myEnum);

枚举是否有什么特别之处阻止我将它们与 ref dynamic 一起使用？

Answer 1

作为引用传递的唯一方法是将 myEnum 转换为动态类型，然后通过引用传递它。我认为我们应该仔细研究生成的 IL，以了解幕后发生的事情。让我们找出原因并分析这个程序:

 enum MyEnum{
     A,B
}

void Main()
{
    MyEnum  myEnum = MyEnum.B; //Assign a variable

    DoSomethingByEnum(myEnum); //Pass myEnum
    DoSomethingDynamicByValue(myEnum); //pass myEnum to a dynamic parameter

    dynamic dyn = myEnum;      //assign myenum to a dynamic variable
    DoSomethingDynamicByRef(ref dyn);    //pass it as a reference
}

MyEnum DoSomethingByEnum(MyEnum a)
{
   return a;
}

dynamic DoSomethingDynamicByValue(dynamic inputObject)
{
   return inputObject;

}

dynamic DoSomethingDynamicByRef(ref  dynamic inputObject)
{
  return inputObject;

} 

首先我们调用 DoSomethingByEnum 按值传递变量 myEnum，然后 DoSomethingDynamicByValue 再次传递 myEnum 但隐式装箱为动态类型。这是在 MSIL 级别发生的情况:

Main:
  IL_0001:  ldc.i4.1    // MyEnum myEnum = MyEnum.B;
  IL_0002:  stloc.0     // myEnum popped from evaluation stack and stored in a local variable 
  IL_0003:  ldarg.0     
  IL_0004:  ldloc.0     // myEnum loaded from local variable at index 0 and passed to the function
  IL_0005:  call        DoSomethingByEnum
  IL_000A:  pop         
  IL_000B:  ldarg.0     
  IL_000C:  ldloc.0     // myEnum
  IL_000D:  box         MyEnum // dynamic dyn = myEnum;  
                       // myEnum Converted from value type to a true object reference of type dynamic
  IL_0012:  call        DoSomethingDynamicByValue

DoSomethingByEnum(MyEnum) 和 DoSomethingDynamicByValue(dynamic) 之间的唯一区别是装箱变量 myEnum(通过创建一个新对象并将数据从值类型复制到新分配的动态对象中来实现)。检查一下 Box Opcode

让我们看看 DoSomethingByEnum(MyEnum) 和 DoSomethingDynamicByValue(dynamic) IL:

 DoSomethingDynamicByValue:
    IL_0000:  nop         
    IL_0001:  ldarg.1     
    IL_0002:  stloc.0      
    IL_0003:  br.s        IL_0005
    IL_0005:  ldloc.0      
    IL_0006:  ret         

DoSomethingByEnum:
    IL_0000:  nop         
    IL_0001:  ldarg.1     
    IL_0002:  stloc.0      
    IL_0003:  br.s        IL_0005
    IL_0005:  ldloc.0     
    IL_0006:  ret         

无论变量的类型如何，这两个函数的 IL 代码完全相同。我们甚至可以拥有任意对象类型，但变量在调用之间传递和共享的方式不会改变。

让我们看看在 DoSomethingDynamicByRef(ref dynamic) 中发生了什么。

继续main方法

Main:
    IL_0018:  ldloc.0     // myEnum
    IL_0019:  box         UserQuery.MyEnum
    IL_001E:  stloc.1     // dyn
    IL_001F:  ldarg.0     
    IL_0020:  ldloca.s    01 // loads the address of dyn onto the stack
    IL_0022:  call        UserQuery.DoSomethingDynamicByRef

DoSomethingDynamicByRef:
   IL_0000:  nop         
   IL_0001:  ldarg.1     
   IL_0002:  ldind.ref   //
   IL_0003:  stloc.0     
   IL_0004:  br.s        IL_0006
   IL_0006:  ldloc.0      
   IL_0007:  ret

此 IL 与前两个示例之间的区别依赖于这两条指令来加载和获取地址:

   ldloca.s    01  // loads the address of dyn onto the stack

   ldind.ref       // Loads the object reference at address addr onto the stack as a type O

我认为不可能传递不同对象类型的地址的原因在上面两条 IL 指令的 MSDN 页面中有解释 ldloca.s和 ldind.ref

Correctly-formed Microsoft Intermediate Language (MSIL) ensures that the ldind instructions are used in a manner consistent with the type of the pointer. The address initially pushed onto the stack must be aligned to the natural size of objects on the machine

希望这可以澄清一点。