unique id last empty

public static void NewDemand(FilterDemand newDemandData)
        {
            using (WMS.DAL.WarehouseLinqDataContext dc = new WMS.DAL.WarehouseLinqDataContext())
            {
                WMS.DAL.Demand Demand = new WMS.DAL.Demand();
                DemandData DemandD = new DemandData();
                List<DemandData> listDemand = new List<DemandData>();
                var log = from de in dc.Demands
                          group de by new { de.ID_Demand } into result
                          select new
                          {
                              id1 = result.Key.ID_Demand,
                              id2 = result.Select(i => i.ID_Demand),
                              y = result.Select(i => i.Year)
                          };

                foreach (var d in log)
                {
                    DemandD.IDDemand = d.id2.Last();
                    listDemand.Add(DemandD);
                }

                string al = DemandD.IDDemand;
                int x = 0;
                string _val1 = al.Substring(1, al.Length - 5);
                x = Convert.ToInt32(_val1) + 1;
                string ff = "";
                ff = "D" + x;
                
                Demand.Month = newDemandData.FilterMonth;
                        Demand.ID_Demand = "D1";
                        Demand.GF = (float)newDemandData.FilterGF;
                        Demand.IND = (float)newDemandData.FilterIndoor;
                        Demand.Year = (int)newDemandData.FilterYear;
                        Demand.Total = Demand.GF + Demand.IND;
                        dc.Demands.InsertOnSubmit(Demand);
                        dc.SubmitChanges();
                
                }
            }