Consider this contrived, trivial example:
var foo = new byte[] {246, 127};
var bar = foo.Cast<sbyte>();
var baz = new List<sbyte>();
foreach (var sb in bar)
{
baz.Add(sb);
}
foreach (var sb in baz)
{
Console.WriteLine(sb);
}
With the magic of Two's Complement, -10 and 127 is printed to the console. So far so good. People with keen eyes will see that I am iterating over an enumerable and adding it to a list. That sounds like ToList
:
var foo = new byte[] {246, 127};
var bar = foo.Cast<sbyte>();
var baz = bar.ToList();
//Nothing to see here
foreach (var sb in baz)
{
Console.WriteLine(sb);
}
Except that does not work. I get this exception:
Exception type: System.ArrayTypeMismatchException
Message: Source array type cannot be assigned to destination array type.
I find this exception very peculiar because
ArrayTypeMismatchException
- I'm not doing anything with arrays, myself. This seems to be an internal exception. Cast<sbyte>
works fine (as in the first example), it's when using ToArray
or ToList
the problem presents itself. I'm targeting .NET v4 x86, but the same occurs in 3.5.
I don't need any advice on how to resolve the problem, I've already managed to do that. What I do want to know is why is this behavior occurring in the first place?
EDIT :
Even weirder, adding a meaningless select statement causes the ToList
to work correctly:
var baz = bar.Select(x => x).ToList();
Okay, this really depends on a few oddities combined:
Even though in C# you can't cast a byte[]
to an sbyte[]
directly, the CLR allows it:
var foo = new byte[] {246, 127}; // This produces a warning at compile-time, and the C# compiler "optimizes" // to the constant "false" Console.WriteLine(foo is sbyte[]); object x = foo; // Using object fools the C# compiler into really consulting the CLR... which // allows the conversion, so this prints True Console.WriteLine(x is sbyte[]);
Cast<T>()
optimizes such that if it thinks it doesn't need to do anything (via an is
check like the above) it returns the original reference - so that's happening here.
ToList()
delegates to the constructor of List<T>
taking an IEnumerable<T>
That constructor is optimized for ICollection<T>
to use CopyTo
... and that's what's failing. Here's a version which has no method calls other than CopyTo
:
object bytes = new byte[] { 246, 127 }; // This succeeds... ICollection<sbyte> list = (ICollection<sbyte>) bytes; sbyte[] array = new sbyte[2]; list.CopyTo(array, 0);
Now if you use a Select
at any point, you don't end up with an ICollection<T>
, so it goes through the legitimate (for the CLR) byte
/ sbyte
conversion for each element, rather than trying to use the array implementation of CopyTo
.
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