linq 升序排序 空值放后面并根据另一个字段进行多重排序

  List<PickingInfo> res = GetDatas();
             var _d = (from e in res
                       select new
                       {
                           aa = e.pickingLibraryName == "" || e.pickingLibraryName == null ? - : ,
                           e
                       }).OrderBy(c => c.e.pickingLibraryName).OrderByDescending(h => h.aa).OrderBy(c => c.e.CommodityBarCode).Select(h => h.e).ToList();//.ToDataTable();
             return JsonConvert.SerializeObject(_d);