Why does .NET foreach loop throw NullRefException when collection is null?

Question

So I frequently run into this situation... where Do.Something(...) returns a null collection, like so:

int[] returnArray = Do.Something(...);

Then, I try to use this collection like so:

foreach (int i in returnArray)
{
    // do some more stuff
}

I'm just curious, why can't a foreach loop operate on a null collection? It seems logical to me that 0 iterations would get executed with a null collection... instead it throws a NullReferenceException. Anyone know why this could be?

This is annoying as I'm working with APIs that aren't clear on exactly what they return, so I end up with if (someCollection != null) everywhere...

Edit: Thank you all for explaining that foreach uses GetEnumerator and if there is no enumerator to get, the foreach would fail. I guess I'm asking why the language/runtime can't or won't do a null check before grabbing the enumerator. It seems to me that the behavior would still be well defined.

Answer 1

Well, the short answer is "because that's the way the compiler designers designed it." Realistically, though, your collection object is null, so there's no way for the compiler to get the enumerator to loop through the collection.

If you really need to do something like this, try the null coalescing operator:

int[] array = null;

foreach (int i in array ?? Enumerable.Empty<int>())
{
   System.Console.WriteLine(string.Format("{0}", i));
}