private Order? SortOrder {
            get
           {
                if (_sortOrder == null)
                {
                    if (ViewState["SortOrder"] != null)
                        _sortOrder = (Order)ViewState["SortOrder"];
                    else
                        _sortOrder = Order.None;
                }
                return _sortOrder;
            }
            set
            {
                _sortOrder = value.Value;
                ViewState["SortOrder"] = value;
            }
        }