Reusing an IEnumerable results in false result, e.g. on .Any()

Question

I'm a little lost in deferred execution land:

I declare an instance of an IEnumerable implementing class

var wordEnumerable = new WordEnumerable(_text);

Then I iterate over it (the first word is "Lorem")

foreach (var word in wordEnumerable)
                    Console.WriteLine(word);

.. which is written to the console.

Now right thereafter in code I do a

Console.WriteLine(wordEnumerable.Any(w => w == "Lorem"));

.. and get a False as output.

Now If I put the .Any(..) part above the foreach loop I do get a true, however the loop does start with the second word.

My expectation was that .Net creates different runtime 'contexts' for each call to an IEnumerable and its underlying IEnumerator so they don't interfere... I wouldn't want to .Reset() it by hand in order to get a proper result?

What am I missing here?

Update:

Link to IEnumerable implementation: https://gist.github.com/814352
Link to IEnumerator implementation: https://gist.github.com/814354
.. and finally the underlying text parser implementation: https://gist.github.com/814358

.. It is basically an IEnumerable that allows me to iterate over the words within a given string.

Could you provide us with more info about WordEnumerable class? I have no problems with creating functionality you want using simple `List`. And if collection was not changed between both code fragments, than your code should work as well, if WordEnumerable class works as expected. — Jarek, Feb 07 '11 at 13:13

score 3 · Accepted Answer · answered Feb 07 '11 at 13:09

Your expectation is correct - Any will call GetEnumerator again to get a fresh IEnumerator<T>. That should be fine if you've implemented IEnumerable<T> correctly. My guess is that your implementation of WordEnumerable is incorrect. Please post the code :)

What happens if you write:

Console.WriteLine("First");
foreach (var word in wordEnumerable)
{
    Console.WriteLine(word);
}

Console.WriteLine("Second");
foreach (var word in wordEnumerable)
{
    Console.WriteLine(word);
}

? The other thing to check is that WordEnumerable implements IEnumerable<string> rather than IEnumerable<object>, otherwise your == check will be a reference identity check.

Oh my.. Thanks Jon! My bad.. I reused the Enumerator :) It's an IEnumerable, so the == comparison does work fine. — Jörg Battermann, Feb 07 '11 at 13:15

Reusing an IEnumerable results in false result, e.g. on .Any()

1 Answers1