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.

Answer 1

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>() );