Where is this ListBox's enumerator being modified in a loop?

Question

The code below works

  void selectColumns_Click(object s, EventArgs e)
    {
        ListBox lBx = getListBox();
        for (int i = 0; i < lBx.Items.Count-1;i++ )
        {
            bool contained = lBx.SelectedItems.Contains(lBx.Items[i]);
            dgView.Columns[lBx.Items[i].ToString()].Visible = contained;
        }
    }

but this does not work- throws error "List that this enumerator is bound to has been modified. An enumerator can only be used if the list does not change."

  void selectColumns_Click(object s, EventArgs e)
    {
        ListBox lBx = getListBox();
        foreach (var item in lBx.Items )
        {
            bool cont = lBx.SelectedItems.Contains(item);
            dgView.Columns[item.toString()].Visible = cont;
        }
    }

Get ListBox method is below:

ListBox getListBox()
{
    return columnsForm.Controls.OfType<ListBox>().First() as ListBox;
}

also the listbox is getting populated by clicking on a hideColumnsButton

void hideBtn_Click(object sender, EventArgs e)
{
    getListBox().Items.Clear();
    foreach(DataGridViewColumn col in dgView.Columns)
    {
        getListBox().Items.Add(col.Name);
    }
    columnsForm.ShowDialog();
}

Just to clarify.. it's not being modified outside of this function is it? — Simon Whitehead
which events are you using modify your listbox? can you update the question with getListBox method code? — Damith

paxdiablo paxdiablo · Accepted Answer · 2013-09-11T04:37:45

There's nothing in your code which explicitly tries to modify the list within the enumeration loop. Calling SelectedItems simply gives you a list of references to the underlying selected items, ot does not modify the list itself.

So the only possibility I can see is that there's another thread of execution (or some code you're not showing us) which is modifying the list outside the scope of that loop.

Where is this ListBox's enumerator being modified in a loop?

1 Answers