if (e.Item is GridEditFormInsertItem || e.Item is GridDataInsertItem)

                {
                    // insert item
                }
                else
                {
                    RadComboBox dropDownUnitListColumn = editedItem["DropDownUnitListColumn"].Controls[0] as RadComboBox;
                    dropDownProductListColumn.Enabled = false;
                    dropDownUnitListColumn.Enabled = false;

                    // edit item
                }