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();

i.e. 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?.)