可空盒装基础类型可以转换为枚举，但盒装枚举类型不能转换为可空类型

问题描述

• 盒装为空的基础类型的可投以枚举，但盒装枚举类型的不能转换可空类型。

同样，

• 盒装为空的枚举的可投以基本类型，但是盒装的基础类型的不能转换以可空的枚举。

的好吧，我知道盒装可空类型是不是来形容它的最好方式，但它的问题的缘故。我知道那就是变得越来越盒装基础值类型。的

我将举例证明这一点。假设我有一个枚举与 INT 作为基础类型。

 枚举性别{男，女} 
  

案例一

 诠释？ I = 1; 
对象o = I; 
性别E =（性别）O; //成功
 
 //但是
 
性别E = Sex.Male; 
对象o = E; 
诠释？ I =（INT）O; //无效转换
  

案例二：

 性爱？ E = Sex.Male; 
对象o = E; 
 INT I =（INT）O; //成功
 
 //但是
 
 INT I = 1; 
对象o = I; 
性别？ E =（性别）O; //无效转换
  

简而言之，

 （枚举）INT？ - >成功
（INT？）枚举 - >反向失败
 
（INT）枚举？ - >成功
（枚举？）INT  - >反向失败
  

或在更简单来说，

Now I do know that once you box a value type, it can be cast back only to the original type. But since by C# rules, a boxed int can be cast to enum and a boxed enum can be cast to int, and a boxed int to int? and a boxed int? to int, I was looking for a consistent understanding of other scenarios as well, ie the ones listed above. But I dont get the logic. For one, I feel if they all failed or if they all succeeded, it made more sense to developers. Two, even the successful casts look a little odd. I mean since a value type can be implicitly cast to its nullable equivalent (and not the other way around), a cast to nullable should anyway succeed, but with the current implementation a nullable type is being successfully cast to non-nullable which can even fail if the former had a null value. Had this whole thing been other way around, it would have been easier comprehending. An example like:

Sex? e = null;
object o = e;
int i = (int)o; //succeeds, but sure to explode on cast

//but

int i = 1;
object o = i;
Sex? e = (Sex?)o; //invalid cast, even though its always a safe cast

Questions:

1. So what C# rule is letting this happen?

2. Is there a simple way I can remember this?

解决方案

I think this is a subtlety of the unbox and unbox.any IL instructions.

From ECMA 335, section III.4.32 (unbox operation - unbox.any is similar)

So for example, in this case:

Sex e = Sex.Male;
object o = e;
int? i = (int?)o;

it fails entirely correctly - because valuetype is Nullable<int> and the value of obj is not a boxed int. The "verifier-assignable-to" part doesn't apply for the Nullable<T> case.

I doubt that any of this behaviour is described in the C# specification, unfortunately - I don't think the unboxing behaviour from "boxed int" to "enum with an underlying type of int" is described, as far as I can see, which is a sort of prerequisite to then including nullability in the mix.

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