只需将Guid.NewGuid（）移入for循环，而不是添加一次。

Simply move Guid.NewGuid() into for loop instead of adding it once.

using (TestDBEntities db = new TestDBEntities())
              {
                maintenance ObjMaintenance = new maintenance
                  {

                   BillNo=TVM.BillNo,
                   BillDate=TVM.BillDate,
                   Amount=TVM.Amount
                   };

                  foreach (var i in TVM.Expenses)
                  {
                      i.MaintenanceId= Guid.NewGuid();
                      ObjMaintenance.Expenses.Add(i);
                  }

                  db.maintenances.Add(ObjMaintenance);
                  //db.serviceproviders.Add(ObjServiceProvider);
                  db.SaveChanges();
                  status = true;
              }