Collection was modified; enumeration may not execute error when removing a ListItem from a LIstBox

Question

I have two ListBoxes, lstAvailableColors and lstSelectedColors. Between each listbox are two buttons, Add and Remove. When a color or colors is selected in lstAvailableCol

Answer 1

You cannot modify an collection while you are using an Enumerator for this collection, what the for each statement does.

You have to loop over the data with a normal for loop and then you can modify the collection, but you must be careful to correctly update the current index if you insert or remove elements. If you just add or remove elements and don't insert some, iterating from the last element to the first will do.

protected void btnAdd_Click(object sender, EventArgs e)
{
    for (Int32 i = lstAvailableColors.Items.Count; i >= 0; i--)
    {
        ListItem item = lstAvailableColors.Items[i];

        if (item.Selected)
        {
            lstSelectedColors.Items.Add(item);
            lstAvailableColors.Items.Remove(item);
        }
    }
}