为什么<enum>的集合无法转换为<int？>？

Question

Why is a collection of enum unable to cast to an int?

enum Test { A = 1, B = 2 };

int? x = (int?)Test.A; // Valid

var collection1 = new[] { Test.A }.Cast<int>().ToList();

// InvalidCastException has thrown (Specified cast is not valid.)    
var collection2 = new[] { Test.A }.Cast<int?>().ToList(); 

Answer 1

The Cast method can only perform boxing/unboxing conversions, reference conversions, and conversions between an enum type and its underlying integral type. The unboxing has to be to the right type though - it can't unbox to a nullable type (unlike the C# conversion).

var collection1 = new[] { Test.A }.Cast<int>()
                                  .Select(x => (int?) x)
                                  .ToList();

For each value, the Cast will unbox from the boxed enum value to the int value, and then the Select will convert the int value to an int? value.

In this case you can also get away with this:

var collection1 = new[] { Test.A }.Select(x => (int?) x)
                                  .ToList();

ie no Cast step. However, that doesn't work if you have an object array instead:

// Fails
var collection1 = new object[] { Test.A }.Select(x => (int?) x)
                                         .ToList();

You can't unbox a boxed enum value to a nullable int value. The Cast version still works in that case, however, as it splits the two steps (unboxing first to int , then converting from int to int? .)

Answer 2

It is because internally Cast() does something like:

object o = Test.A;

int i = (int)o; // This is valid.
int? ni = (int?)o; // This is not valid!

You would have to first Cast() to an int:

var collection3 = new[] { Test.A }.Cast<int>().ToList().Cast<int?>().ToList();

or not use Cast() at all:

var collection4 = new object[] { Test.A, null }.Select(i => i == null ? null : (int?)(int)i).ToList();

Answer 3

It wouldn't work because we need to understand what is enum and what is int? (nullable int)

int? is not really an int it is a generic instance of Nullable, - The syntax T? is shorthand for System.Nullable, where T is a value type. The two forms are interchangeable. Link

Enum The default underlying type of the enumeration elements is int. Link

Therefore an implicit conversion between int and enum exists. But implicit conversion between an enum(int) to Nullable (which is entirely different reference type) doesnt exist. That is why you see an InvalidCastException exception.

Where as in the first int? x = (int?)Test.A; int? x = (int?)Test.A; statement you are forcing the explicit conversion to either box the value type and get an integer or get a null, which is works because boxing an enum into object then unboxing it as int? is returning an int.

Answer 4

我不知道为什么Cast<>()不起作用，但你可以使用Select()来做你想要的。

var collection = new[] { Test.A }.Select(item => (int?)item).ToList();