public List<InvItem> invitems;  //start of indexer
public InvItem this[int i]
{
    get
    {
        try
        {
            Validator.IsWithinRange(i);
        }
        catch (ArgumentOutOfRangeException)
        {
            throw;
        }
        return invitems[i];
    }
    set
    {
        invItems[i] = value;
    }
}
private void FillItemListBox()
{
	List<InvItem> items = new List<InvItem>();
	lstItems.Items.Clear();
	lstItems.DataSource = items;
}

For the life of me, I can't figure out where I am going wrong. Thanks for any help!