BAL.Manager.CarManager.ChangeCarToMain C# (CSharp) Method

        public string ChangeCarToMain(int carId, int userId)
        {
            if (carId <= 0 || userId <= 0)
            {
                return "Error";
            }
            // We need cars that are currently being used by current driver.
            var usedCar = uOW.CarRepo.Get(x => x.isMain == true & x.UserId == userId).FirstOrDefault(); // find a car with status set as true, which are used by our driver
            if (usedCar != null) // if such car exists
            {
                if (usedCar.Id == carId) // and if this car id match input parameter
                {
                    return MakeACarMain(usedCar); // change this car`s id
                }
                else // if it`s not, if its other car, change its status to false and change status of car that we need
                {
                    uOW.CarRepo.SetStateModified(usedCar);
                    usedCar.isMain = false;
                    uOW.Save();
                    var message = MakeACarMain(carId, userId);
                    return message;
                }
            }
            else // if there are no cars with status as true
            {
                var message = MakeACarMain(carId, userId);
                return message;
            }
        }