Question

Possible Duplicate:
Puzzling Enumerable.Cast InvalidCastException

Hi,

I just noticed something quite strange with the Enumerable.Cast<T> extension method… It seems that it can’t cast from int to long, even though this cast is perfectly legal.

The following code fails with an InvalidCastException :

        foreach (var item in Enumerable.Range(0,10).Cast<long>())
        {
            Console.WriteLine(item);
        }

But this code, which I assumed to be equivalent, does work :

        foreach (var item in Enumerable.Range(0,10).Select(i => (long)i))
        {
            Console.WriteLine(item);
        }

Can anyone explain that behavior ? I looked at the code of the Cast method with Reflector, but Reflector can’t interprete iterator blocks, so it’s pretty hard to understand…

Answer 1

The relevant line in Cast:

 this.<>2__current = (TResult)this.<obj>5__ab;

We can mimic this using the following code:

int foo = 1;
long bar = Cast<long>(foo); //oh noes!

T Cast<T>(object input)
{
    return (T)input;
}

Which also fails. The key here is that at the point of cast, it’s an object. Not an int. This fails because we can only unbox from an object to the exact type we want. We are going from object – which could be a boxed long, but it’s not. It’s a boxed int. Eric Lippert discussed this on his blog:

we’ve decided that unboxing can only unbox to the exact type. If you want to call the slow method that does all that goo, it’s available – you can always call Convert…

In your code which works, you’re not dealing with a boxed int (an object), you’ve got an int.