Collection was modified; enumeration operation may not execute in ArrayList [duplicate]

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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)

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