InvalidOperationException after removing an element in an arrayList

Question

Hi I am trying to remove all numbers that are divisible by two from the arrayList.The probleme is that after one element is removed I get an InvalidOperationException.

   private ArrayList RemoveDivTwo(ArrayList list) {
        int count = 0;
        foreach(int i in list){
            if ((i > 2) && (i % 2 == 0)) {
                list.RemoveAt(count); 
            }
            count++;
        }

        return list;
    }

How can I solve this problem so I wont't get an exception and be able to remove all elements divisible by two?

This is a common one. Remember that you can't add/remove items to/from a collection if you're iterating with a `foreach`. — s.m., Oct 20 '12 at 11:26
If you are thinking,why is this an invalid op,Please read this http://stackoverflow.com/questions/1124221/modifying-collection-when-using-a-foreach-loop-in-c-sharp — Prabhu Murthy, Oct 20 '12 at 11:32
`ArrayList` is not a preferred collection type, unless you are working with legacy 1.1 code. Use a [`List`](http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx) instead. Using a `List` your code could be rewritten to `list.RemoveAll(i => i > 2 && i % 2 == 0)` — Paolo Moretti, Oct 20 '12 at 11:53

Anirudh Ramanathan · Accepted Answer · 2012-10-20T11:20:33.563

3

Try iterating over it this way.

for(int i = 0; i < list.Count(); i++){
   if ((list[i] > 2) && (list[i] % 2 == 0)) {
                list.RemoveAt(i); 
                i--; //as offsets have shifted by one due to removal
            }
}

You are no longer iterating over the list. So this should work.

edited Oct 20 '12 at 11:20

answered Oct 20 '12 at 11:13

Anirudh Ramanathan

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the only problem with this is that I am using an arrayList and I can not get the elements threw indexes – Nistor Alexandru Oct 20 '12 at 11:42
Yes you can; you'll just have to cast them back to int. This is another reason to use `List` if you possibly can. – Quick Joe Smith Oct 20 '12 at 14:06

score 1 · Answer 2 · answered Oct 20 '12 at 11:36

The exception is thrown because, foreach loop calls collectionName.GetEnumerator method before it starts iterating through the list of items. GetEnumerator is not called again unless you start a new foreach loop on the collection. The list cannot be modified inside the foreach loop, it is meant only for read-only operations on the list.

You may use for loop for iterating as well as modifying the elements from the list.

score 1 · Answer 3 · answered Oct 20 '12 at 14:04

1

I wouldn't even bother removing elements. Just return the elements you do want as a new list:

List<int> RemoveDivTwo(List<int> list) {
    return list.Where(i => i % 2 == 1 || i <= 2).ToList();
}

answered Oct 20 '12 at 14:04

Quick Joe Smith

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InvalidOperationException after removing an element in an arrayList

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