not in query using linq..

if (dsExistingBillGeneration != null && dsExistingBillGeneration.Tables[0].Rows.Count > 0)
                {
                    List<BillGenerationLibrary> lstBillgen = new List<BillGenerationLibrary>();

                    for (int i = 0; i < dsExistingBillGeneration.Tables[0].Rows.Count; i++)
                    {

                     
                        var billdata = from lstbilldata in listBillGenerationLibrary where lstbilldata.IsDirectBilling == 1 && (lstbilldata.ApartmentId != Convert.ToInt32(dsExistingBillGeneration.Tables[0].Rows[i]["ApartmentId"])) select lstbilldata;
                        lstBillgen = billdata.ToList();

                    }
                    returnGroupId = objBillGenerationBussinessRules.Insert_ServiceManagement_BillGeneration(lstBillgen);
                }