Removing an Item in a list

Question

I'm programming in c#. I'm trying to remove an item from a List<> but when I remove the item I get this exception error:

An exception of type 'System.InvalidOperationException' occurred in mscorlib.dll but was not handled in user code

Additional information: Collection was modified; enumeration operation may not execute.

Here is my code:

foreach (Target t in targetList)
{
    if (t.CalculateDistance(t.EndX, t.EndY) <= 5)
    {
        targetList.Remove(t);
    }
}

I get exception on first line. Why do I see this error? or How can I fix it?

You can either: use a `for` loop, which I don't recommend, your `iterator` will likely end up out-of-range at some point; create a second `List` or `Array` to hold values you wish to remove, then add each value you wish to remove to the `List` or `Array` and after complete, iterate through that `List` or `Array` and remove those from the original `List` or `Array`. — Der Kommissar, Apr 24 '15 at 20:35
`Your iterator will likely end up out-of-range at some point` not if you go backwards — Ňɏssa Pøngjǣrdenlarp, Apr 24 '15 at 20:36
@Plutonix That is true, though not recommended. As I said, *likely*. OP should refer to the LINQ query in the first answer here. — Der Kommissar, Apr 24 '15 at 20:39

Luc · Answer 1 · 2015-04-24T20:56:19.000

12

The problem you face is that you can't modify the collection you itterate thru. You could solve this by using linq:

targetList.RemoveAll(t => t.CalculateDistance(t.EndX, t.EndY) <= 5);

edited Apr 24 '15 at 20:56

answered Apr 24 '15 at 20:36

Luc

1,491
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Valid answer, but it would be helpful if you described WHY the OP's solution wasn't working. That was the first part to his question. – Rufus L Apr 24 '15 at 20:49

score 1 · Answer 2 · edited Apr 24 '15 at 20:47

1

Try this:

var needdelete = new List<Target>();

foreach (Target t in targetList)
{
    if (t.CalculateDistance(t.EndX, t.EndY) <= 5)
    {
        needdelete.Add(t);           
    }
}

targetlist.RemoveRange(needdelete);

edited Apr 24 '15 at 20:47

Rufus L

36,127
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answered Apr 24 '15 at 20:40

Uğurhan Gül

19
4

1

This answer is ugly compared to the other, but does have the desired effect. – Der Kommissar Apr 24 '15 at 20:42
Valid answer, but it would be helpful if you described WHY the OP's solution wasn't working. That was the first part to his question. – Rufus L Apr 24 '15 at 20:49

Removing an Item in a list

2 Answers2