Why Enumerable.Cast raises an InvalidCastException?

41
votes

If I can implicitly cast an integer value to a double, like:

int a = 4;    
double b = a;
// now b holds 4.0

Why can I not do this:

int[] intNumbers = {10, 6, 1, 9};    
double[] doubleNumbers2 = intNumbers.Cast<double>().ToArray();

I get a "Specified cast is not valid" InvalidCastException exception.

Doing the opposite (casting from double to int) results in the same error.

What am I doing wrong?

2
c#castingienumerable
Small point of pedantry on the "and vice versa" - the conversion from double to integer is explicit, not implicit. – Jon Skeet
removed this "pedantry" point. Thanks – outlookrperson
Possible duplicate of Puzzling Enumerable.Cast InvalidCastException – Edward Brey

2 Answers

59
votes

Well, you have incorrect expectations of Cast, that's all - it's meant to deal with boxing/unboxing, reference and identity conversions, and that's all. It's unfortunate that the documentation isn't as clear as it might be :(

The solution is to use Select:

doubleNumbers2 = intNumbers.Select(x => (double) x).ToArray();
9
votes

To add to Jon's answer cast is mainly useful for objects that implement IEnumerable but nothing else. Take XmlNodeList for example. If you don't have the luxury of using System.Xml.Linq namespace you can use Cast<XmlElement> to write some nice LINQ queries against it.

var result = xmlNodeList
    .Cast<XmlElement>()
    .Select(e=> e.GetAttribute("A") + e.GetAttribute("B"))
    .ToArray();