How do I loop through items in a list box and then remove those item?

Question

I'm getting the error below when trying to loop through a listbox and then remove the item.

List that this enumerator is bound to has been modified. An enumerator can only be used if the list does not change.

foreach (string s in listBox1.Items)
{
    MessageBox.Show(s);
    //do stuff with (s);
    listBox1.Items.Remove(s);
}

How can I remove the item and still loop through the contents?

Bowler

Answer 1

Do you want to remove all items? If so, do the foreach first, then just use Items.Clear() to remove all of them afterwards.

Otherwise, perhaps loop backwards by indexer:

listBox1.BeginUpdate();
try {
  for(int i = listBox1.Items.Count - 1; i >= 0 ; i--) {
    // do with listBox1.Items[i]

    listBox1.Items.RemoveAt(i);
  }
} finally {
  listBox1.EndUpdate();
}

Implosion

Me too Marc. +1 to counter act the injustice of it. – Hardshell

I'd love to know the reason for downvotes. Both for me and you! +1 – Misesteem

@Binary Worrier: In fact upvotes do not really neutralize downvotes, due to 200 points reputation limit. – Misesteem

@MehrdadAfshari But they do neutralize the negative effect they have on the answer's score. +1! :) – Frowsty

Answer 2

Everyone else has posted "going backwards" answer, so I'll give the alternative: create a list of items you want to remove, then remove them at the end:

List<string> removals = new List<string>();
foreach (string s in listBox1.Items)
{
    MessageBox.Show(s);
    //do stuff with (s);
    removals.Add(s);
}

foreach (string s in removals)
{
    listBox1.Items.Remove(s);
}

Sometimes the "work backwards" method is better, sometimes the above is better - particularly if you're dealing with a type which has a RemoveAll(collection) method. Worth knowing both though.

Bermuda

-1. listBox1.Items might contain objects other than string, in that case InvalidCastException will be thrown. – Misesteem

If that were the case, the foreach loop in the sample code in the question would already have blown up. I was making the same assumption as the question did, which I think is pretty reasonable. – Bermuda

Yeah, I noticed it. It was just fun to downvote you for a reason for a minutes ;) – Misesteem

At least you gave a reason :) – Bermuda

One day my rep will be greater than Jons and Mehdrad's combined. Muhahahaha! Assuming that people are required to give a reason for down-voting, of course. – Nomography

Answer 3

Here my solution without going backward and without a temporary list

while (listBox1.Items.Count > 0)
{
  string s = listBox1.Items[0] as string;
  // do something with s
  listBox1.Items.RemoveAt(0);
}

Monopetalous

@Fernando68 as the count of the ListBox is decreased every time an item is removed, the while condition will eventually equate to false and the loop will end. – Idiomatic

Answer 4

You have to go through the collection from the last item to the first. this code is in vb

for i as integer= list.items.count-1 to 0 step -1
....
list.items.removeat(i)
next

Marking

Answer 5

Jefferson is right, you have to do it backwards.

Here's the c# equivalent:

for (var i == list.Items.Count - 1; i >= 0; i--)
{
    list.Items.RemoveAt(i);
}

Quean

@Fernando68 Because back in 2008, there were people who can't translate from VB to C# :D – Quean

Answer 6

How about:

foreach(var s in listBox1.Items.ToArray())
{
    MessageBox.Show(s);
    //do stuff with (s);
    listBox1.Items.Remove(s);
}

The ToArray makes a copy of the list, so you don't need to worry about it changing the list while you are processing it.

Desquamate

Answer 7

while(listbox.Items.Remove(s)) ; should work, as well. However, I think the backwards solution is the fastest.

Cenesthesia

Answer 8

You can't make modification to the collection being iterated within the ForEach block.

A quick fix is to iterate over a copy of the collection. An easy way to make this copy is through the ArrayList constructor. The DataRowView objects in the copied collection will refer to, and be able to modify, the same underlying data as your code.

For Each item As DataRowView In New System.Collections.ArrayList(lbOrdersNeedToBeVoided.Items)

please read http://social.msdn.microsoft.com/Forums/en-AU/vbgeneral/thread/b4d1f649-d78a-4e5b-8ad8-1940e3379bed

Us