C# List insert in foreach

Question

I have the following loop:

List<Reminders> reminds = new List<Reminders>();
//...
foreach (Reminders remind in reminds)
{
    //....
    reminds.Insert(id, new Reminders() { Title = remind.Title, Content = remind.Content, Checked = true });
}

However, an error occurs in the foreach loop.

foreach (Reminders remind in reminds)

If I remove the reminds.Insert statement, the error no longer occurs. I'm trying to update some entires inside of the foreach loop. What's causing the error?

your can't modify a collection while iterating over it using `foreach`, what exactly are you trying to do ? — Habib, Dec 18 '14 at 16:37
Check this: http://stackoverflow.com/questions/759966/what-is-the-best-way-to-modify-a-list-in-a-foreach-loop — Michael B, Dec 18 '14 at 16:37
why you don't use Add method. Do you need to insert the elements in an specific order? — ocuenca, Dec 18 '14 at 16:39
I don't need to use an insert if you can update specific entries in a different way . — GoOx, Dec 18 '14 at 16:41
Lets first we understand what you want to do to update something in your collection or to insert NEW item in the collection. — mybirthname, Dec 18 '14 at 16:52
If it did work your loop would run forever because for each iteration you list gets one item loner. Why are you inserting a record for every record in the list? — Ben Robinson, Dec 18 '14 at 16:53

score 5 · Answer 1 · answered Dec 18 '14 at 16:51

5

Change your Code to this:

List<Reminders> reminds = new List<Reminders>();
...
foreach (Reminders remind in reminds.ToList())
{
    ....
    reminds.Insert(id, new Reminders() { Title = remind.Title, Content = remind.Content, Checked = true });
}

Please note the .ToList() behind reminds.

Explanation: You are not allowed to modify a collection while it is enumerated in an foreach. The .ToList() will create another temporary collection which will be constrant during the foreach loop.

answered Dec 18 '14 at 16:51

DrKoch

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While this could have memory and performance implications, if you have a reasonable idea of your list size then it's rather clever. Not sure if a junior programmer is going to see the nuance though. – Pale Ale Mar 25 '15 at 14:09

ocuenca · Answer 2 · 2014-12-18T16:50:23.850

3

It's true, you are iterating in the same list. You need to create a temporary list, and add the elements in that temp list in the cycle. After, when the foreach finish, you need to use the method AddRange:

reminds.AddRange(tempList);

edited Dec 18 '14 at 16:50

answered Dec 18 '14 at 16:43

ocuenca

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t3chb0t · Accepted Answer · 2014-12-18T17:11:19.353

1

If you want to update some entries then you shoudn't add new ones just set the property Checked to true of each entry:

List<Reminders> reminds = new List<Reminders>();
...
foreach (Reminders remind in reminds)
{
    ....
    remind.Checked = true;
}

You must not modify the list that you are iterating.

edited Dec 18 '14 at 17:11

answered Dec 18 '14 at 16:50

t3chb0t

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thanks! I didn't think it was that simple. – GoOx Dec 18 '14 at 17:01

score 0 · Answer 4 · answered Dec 18 '14 at 16:37

0

No, You can't insert into an item while iterating through it. Your results will be wrong. You will need to create a temporary list and do an AddRange() after.

answered Dec 18 '14 at 16:37

Kevin

1

C# List insert in foreach

4 Answers4