Exception when Interating through ListBox.Items

Question

If no item is selected in the ListBox, then the code works fine.

If at least one item is selected in the ListBox, the foreach iteration breaks after the first item is evaluated. The exception is an InvalidOperationException and the detail shows Items collection has been modified.

foreach (object item in listBoxFiles.Items) //InvalidOperationException occurs
{
    if (listBoxFiles.SelectedItems.Contains(item)) 
    {
        //do nothing
    }
}

Edit: I was looking for something like ListBoxItem.IsSelected but it does not exist.

Are you sure //do nothing does nothing to the item collection? — Robin Maben
– Robin Maben, Commented Jul 23, 2012 at 6:45
Yes, in fact after removing all the codes inside the if-clause the error is still reproducible. — kennyzx
– kennyzx, Commented Jul 23, 2012 at 6:56
If I select an item and then deselect it, the exception is thrown. If I don't select/unselect any item, the exception will not be thrown. Strange behaviour, I think this is a bug in the WinForm library. — kennyzx
– kennyzx, Commented Jul 23, 2012 at 7:48
I created a form with a listbox and a button. The LB gets populated with some items and the click-Handler for the button executes your code above. The interation throws even if I never selected any items. — EricSchaefer
– EricSchaefer, Commented Jul 23, 2012 at 8:06

EricSchaefer · Accepted Answer · 2012-07-23 07:35:24Z

3

I can reproduce the problem. The access to SelectedItems seems to be changing Items, not the call to Contains. It should not do that. I have no explanation at the moment.

Workaround:

If you check if item is contained in SelectedItems you could right away iterate over SelectedItems instead. Another alternative would be to copy SelectedItems before the iteration like this:

List<object> selectedItems = new List<object>();
selectedItems.AddRange( listBoxFiles.SelectedItems.OfType<object>() );

edited Jul 23, 2012 at 7:35

answered Jul 23, 2012 at 6:11

EricSchaefer

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5 Comments

Glen Hughes Over a year ago

No need to make a full copy... you could just create a new reference outside of the iteration: ListBox.SelectedObjectCollection selectedItems = listBoxFiles.SelectedItems;

kennyzx Over a year ago

@drumboog - new reference does not work. I have to create a full copy (Eric's workaround).

Robin Maben Over a year ago

Yes, a reference would still work on the same collection. (as expected)

EricSchaefer Over a year ago

@Robin Maben - A reference does not work. The get-accessor of SelectedItems seems to be doing something odd.

EricSchaefer Over a year ago

And that is not an assumption or an opinion, a did actually try it.

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