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?

Answer 1

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);
  }
}

Answer 2

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);

Answer 3

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