Collection was modified; enumeration operation may not execute - why?

Question

I\'m enumerating over a collection that implements IList, and during the enumeration I am modifying the collection. I get the error, \"Collection was modified; enumeration

Answer 1

While others have described why it's invalid behaviour, they haven't offered up a solution to your problem. While you may not need a solution, I'm going to provide it anyway.

If you want to observe a collection that is different to the collection that you are iterating over, you must return a new collection.

For instance..

IEnumerable sequence = Enumerable.Range(0, 30);
IEnumerable newSequence = new List();

foreach (var item in sequence) {
    if (item < 20) newSequence.Add(item);
}

// now work with newSequence

This is how you should be 'modifying' collections. LINQ takes this approach when you want to modify a sequence. For instance:

var newSequence = sequence.Where(item => item < 20); // returns new sequence