Safely remove list from list in loop

Question

I am iterating through all the list in mainlist and if a specific condition occur. I want to remove that specific list.

It is not working.

for (int i = 0; i < mainList.Count; i++)
{
     if (mainList[i].Dead == true) //
     {
          mainList.removeAt(i);//what will be affect on mainList.Count;
     }
 }

possible duplicate of [How to modify or delete items from an enumerable collection while iterating through it in C#](http://stackoverflow.com/questions/308466/how-to-modify-or-delete-items-from-an-enumerable-collection-while-iterating-throu) — Alastair Pitts, Aug 15 '11 at 06:43

score 10 · Accepted Answer · answered Aug 15 '11 at 06:41

10

mainList.RemoveAll(x => x.Dead);

answered Aug 15 '11 at 06:41

Cheng Chen

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I'm starting to enjoy the taste of lamb – Paul Bellora Aug 15 '11 at 06:42

score 4 · Answer 2 · answered Aug 15 '11 at 06:42

The way you constructed your for loop, it will only work if mainList.Count stays constant, but you are removing elements during your loop. Do you understand why that is a problem? Whenever you remove an element you end up skipping the check of the element after it.

I think you could use foreach and mainList.remove but I'm not sure. Another solution is to just make your loop count downwards:

for (int i = mainList.Count-1; i >= 0; i--)
{
    if (mainList[i].Dead == true)
    {
         mainList.removeAt(i);
    }
}

No you shouldn't use a foreach and remove, that will alter the list which you are iterating over which is a bad thing. — Tomas Jansson, Aug 15 '11 at 08:11

score 0 · Answer 3 · answered Aug 15 '11 at 06:41

0

You will not be able to do that since you are altering the length of the list when you are doing so. I would do it like this instead:

var newList = mainList.Where(y => !y.Dead).ToList();

answered Aug 15 '11 at 06:41

Tomas Jansson

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this doesn't remove the elements, though, it would require a further step to remove all the items in newList from mainList... – shelleybutterfly Aug 15 '11 at 06:46
no, it doesn't remove the item it creates a new list. The accepted solution does the samething behind the scene but with a delayed execution. – Tomas Jansson Aug 15 '11 at 08:11
well, it looks like the accepted solution is iterating backward through the list, and using RemoveAt(i) to remove the items, so I'm not sure I understand what you mean. (unless you mean the other proposed possible solution which was incomplete and commented on as a bad idea.) regardless, no offense was intended, I was just pointing out that your response does not answer the question, figuring you had forgotten to include the final step. – shelleybutterfly Aug 15 '11 at 08:31

score 0 · Answer 4 · edited May 23 '17 at 12:32

0

How to modify or delete items from an enumerable collection while iterating through it in C#

http://bytes.com/topic/c-sharp/answers/238241-how-do-you-modify-array-while-iterating-through

edited May 23 '17 at 12:32

Community

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answered Aug 15 '11 at 06:42

jimjim

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Safely remove list from list in loop

4 Answers4