public string RegisterCarRental(RentalRegistration rentalRegistration)
 {
     Console.WriteLine("RegisterCarRental");
     using (DataClassesRentalDataContext ctx = new DataClassesRentalDataContext())
     {
         Rental rentalToInsert;
         rentalToInsert = new Rental();
         rentalToInsert.CustomerID = rentalRegistration.CustomerID;
         rentalToInsert.CarID = rentalRegistration.CarID;
         rentalToInsert.Comments = rentalRegistration.Comments;
         ctx.Rental.InsertOnSubmit(rentalToInsert);
         ctx.SubmitChanges();
     }
     return "OK";
 }
 partial void UpdateRental(Rental instance);
 partial void DeleteRental(Rental instance);
 partial void InsertRental(Rental instance);