C# How do I do I loop through items in a list box and then remove that item?

Question

Hi All,

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?

Thanks

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();
}

Answer 2

for (int i = 0; i < listBox1.Items.Count; ++i)
{
   // ...
   listBox1.Items.RemoveAt(i--);
}

Answer 3

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

Answer 4

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);
}

Answer 5

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.

Answer 6

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);
}

Answer 7

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.