Possible to modify a List while iterating through it?

Question

I have the following:

foreach (var depthCard in depthCards)
{
    var card = InternalGetCard(db, depthCard.CardId);
    var set = InternalGetSet(db, (int)card.ParentSetId);
    var depthArray = InternalGetDepthArrayForCard(db, set.SetId);
    foreach (var cardToUpdate in set.Cards)
    {
        // do stuff
        SaveChanges(db);
        // since I already took care of it here, remove from depthCards
        depthCards.Remove(depthCardToUpdate);
    }
}

This isn't working though because I'm modifying the collection in the middle of a loop. Is there some type of collection that does allow this type of access?

I don't want to ToList() the depthCards because I already have them and I want to modify that list as I'm iterating. Is this possible?

score 34 · Accepted Answer · answered Jul 20 '13 at 22:11

34

It's possible, the trick is to iterate backwards:

for (int i = depthCards.Count - 1; i >= 0; i--) {
  if (depthCards[i] == something) { // condition to remove element, if applicable
     depthCards.RemoveAt(i);
  }
}

answered Jul 20 '13 at 22:11

Óscar López

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Tim Schmelter · Answer 2 · 2021-11-11T08:10:21.227

8

You can iterate backwards with a for-loop

for (int i = depthCards.Count - 1; i >= 0; i--)
{
    depthCards.RemoveAt(i);
}

or if you just want to remove items on a condition, use List.RemoveAll:

depthCardToUpdate.RemoveAll(dc => conditionHere);

edited Nov 11 '21 at 08:10

answered Jul 20 '13 at 22:12

Tim Schmelter

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Could always add a `Where()` statement in before doing the `ForEach()` to narrow down the results before removing them, if its a conditional removal. – Bob. Jul 21 '13 at 01:57
@Bob.: No, because then it wouldn't be a `List` anymore but a `IEnumerable` which doesn't support the `List.ForEach` method. You could only use `foreach` then, but then it's not possible to modify the list in the loop. – Tim Schmelter Jul 21 '13 at 09:56
Isn't that what `ToList()` is for? – Bob. Jul 21 '13 at 16:24
No, ToList creates a new list. So you,re doubling the memory consumption(in the best case). – Tim Schmelter Jul 21 '13 at 16:45
The second code snippet still would potentially crash as it is forbidden to modify the underlying collection while iterating with ForEach. See https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.foreach?view=netframework-4.8#remarks – Vringar Nov 10 '21 at 16:52
1

@Vringar: You're right, might not work. Docs say: "Modifying the underlying collection in the body of the Action delegate is not supported and causes undefined behavior." Deleted that approach. Thanks – Tim Schmelter Nov 11 '21 at 08:10

score 1 · Answer 3 · answered Jul 20 '13 at 22:08

1

You can create a custom enumerator that handles this for you. I did this once and it was a bit tricky but worked after some finesse.

See: http://www.codeproject.com/Articles/28963/Custom-Enumerators

answered Jul 20 '13 at 22:08

Haney

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Possible to modify a List while iterating through it?

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