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C# Collection was modified; enumeration operation may not execute.

Answer 1

+1 A:

Don't modify the list inside of a loop which iterates through the list.

David Lively 2010-01-07 22:33:16

Snap! So, how should i be doing it?

Ricardo 2010-01-07 22:33:56

Use a regular loop instead (for/while). The problem is using a 'foreach' statement (i.e. an enumerator).

Noon Silk 2010-01-07 22:34:45

@silky f you're using a for() or while() to iterate through the collection, be sure to do it from the back/top - starting at collection.count-1 and going down to 0. Otherwise you'll hit the same problem.

David Lively 2010-01-23 07:35:02

Answer 2

+19 A:

You are removing the item during a foreach, yes? Simply, you can't. There are a few common options here:

use List<T> and RemoveAll with a predicate

iterate backwards by index, removing matching items

for(int i = list.Count - 1; i >= 0; i--) {
    if({some test}) list.RemoveAt(i);
}

use foreach, and put matching items into a second list; now enumerate the second list and remove those items from the first (if you see what I mean)

Marc Gravell 2010-01-07 22:34:22

+1, too quick for me :)

Glenn Condron 2010-01-07 22:35:22

I see what you mean... i'll try to do it

Ricardo 2010-01-07 22:35:33

if the collection isn't that big, a simple .ToArray() and enumerating that instead might be good enough as well.

Lasse V. Karlsen 2010-01-07 22:36:42

Thank you my good sir

Ricardo 2010-01-07 22:50:18

That's pretty neat, ive always been iterating forwards and conditionally decrementing `i` so that next time it increments it will point to the item after the deleted one. This avoids the extra step.

Igor Zevaka 2010-01-07 22:57:55

Backwards iteration works fine. I'd avoid using .ToArray() as it adds an unnecessary loop to your algorithm (one to copy the list to an array, one to remove the items you're interested in).

David Lively 2010-01-08 17:50:07

Answer 3

A:

Instead of foreach(), use a for() loop with a numeric index.

Seva Alekseyev 2010-01-07 22:35:21

You *must* count backwards if you use this approach though.

Rob Levine 2010-01-07 22:39:06

@Rob - not *must*, but it is trickier if you go forwards; more ways to get it wrong and end up off-by-one.

Marc Gravell 2010-01-07 22:45:50

Answer 4

A:

One way is to add the item(s) to be deleted to a new list. Then go through and delete those items.

ozczecho 2010-01-07 22:35:41

Answer 5

+2 A:

Here's an example (sorry for any typos)

var itemsToRemove = new ArrayList();  // should use generic List if you can

foreach (var item in originalArrayList) {
  if (...) {
    itemsToRemove.Add(item);
  }
}

foreach (var item in itemsToRemove) {
  originalArrayList.Remove(item);
}

OR if you're using 3.5, Linq makes the first bit easier:

itemsToRemove = originalArrayList
  .Where(item => ...)
  .ToArray();

foreach (var item in itemsToRemove) {
  originalArrayList.Remove(item);
}

Replace "..." with your condition that determines if item should be removed.

Will 2010-01-07 22:46:47

If you're using "generic List" (which I interpret as `List<T>`) then just use `list.RemoveAll(item => ...)`.

Marc Gravell 2010-01-07 23:26:32

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C# Collection was modified; enumeration operation may not execute.

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