枚举[]是IEnumerable< int>在通用方法中返回true [英] enum[] is IEnumerable<int> returns true in a generic method
问题描述
这是对这个问题的追踪：演员LT; INT> .Cast<诠释>应用于通用枚举收集结果无效转播例外
This is a followup to this question: Cast<int>.Cast<int?> applied on generic enum collection results in invalid cast exception
enum Gender { Male, Female }
Gender g = Gender.Male;
bool b = g is int; // false, alright no issues
b = new[] { g } is IEnumerable<int>; // false, alright no issues
b = Is<Gender, int>(g); //false, alright no issues
b = Is<Gender[], IEnumerable<int>>(new[] { g }); // true, why on earth !!!
static bool Is<S, T>(S s)
{
return s is T;
}
为什么 Gender []是IEnumerable< int> ;在通用情况下返回 true 特别是当它们不兼容时？
Why is that Gender[] is IEnumerable<int> returns true in the generic case? Especially when they are not type compatible?
IEnumerable<int> c = new[] { Gender.Male }; //not compilable
在我链接的问题中，我认为这个问题是相关问题的关键。
It had tripped me in the question I linked! I think this question is the crux of the issue of the linked question.
对于有兴趣的人来说，这是一个角落数组（不是真的枚举）。按照 Eric Lippert的博客文章在答案中了解更多的这种边缘案例。例如， List< T> 不会发生这种情况：
For someone interested, this is a corner case with arrays (not really enums). Follow Eric Lippert's blog article in the answer to know more of this edge case. This doesn't happen with List<T> for instance:
b = Is<List<Gender>, IEnumerable<int>>(new List<Gender> { g }); // false, rightly
推荐答案
我认为这是那些的C＃定义不同于CLI的定义 isinst ，这显然将枚举作为它们的基础检查数组分配兼容性时键入。 （Eric Lippert写了一个 uint [] 被CLI视为一个 int [] 不是C＃;我怀疑这里也有相同的解释）你甚至不需要泛型来证明：
I think this is one of those cases where the C# definition of is differs from the CLI's definition of isinst, which evidently treats enums as their underlying base type when checking for array assignment compatibility. (Eric Lippert wrote a blog post that explains why uint[] is treated as an int[] by the CLI but not by C#; I suspect the same explanation applies here.) You don't even need generics to demonstrate:
Gender g = Gender.Male;
Console.WriteLine(new[] { g } is IEnumerable<int>); // False
Console.WriteLine((object)new[] { g } is IEnumerable<int>); // True
第一个是表达式由于C＃编译器知道性别[] isn被优化为 false 't an IEnumerable< int> 。第二个表达式生成运行时评估的 isinst 指令。引用Eric Lippert：
The first is expression is optimized to false at compile time because the C# compiler "knows" Gender[] isn't an IEnumerable<int>. The second is expression generates an isinst instruction which is evaluated at run time. Quoting Eric Lippert:
不幸的是，C＃和CLI规范在这个小点上不同意，但是我们愿意以不一致的方式生活。
It is unfortunate that C# and the CLI specifications disagree on this minor point but we are willing to live with the inconsistency.
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