void SortOrderChanged(object sender, EventArgs e)
        {
            TreeColumn changed = sender as TreeColumn;

            // Only one column at a time can have a sort property set
            if (changed.SortOrder != SortOrder.None)
            {
                foreach (TreeColumn col in this)
                {
                    if (col != changed)
                    {
                        col.SortOrder = SortOrder.None;
                    }
                }
            }
            _treeView.UpdateHeaders();
        }