public bool TryUpdate(Car car) {

            Car oldCar;
            if (_carsDictionary.TryGetValue(car.Id, out oldCar)) {

                return _carsDictionary.TryUpdate(car.Id, car, oldCar);
            }

            return false;
        }
        public Car Add(Car car) {

            lock (_incLock) {

                car.Id = _nextId;
                _carsDictionary.TryAdd(car.Id, car);
                _nextId++;
            }

            return car;
        }
        public HttpResponseMessage PostCar(Car car) {

            _carsContext.Add(car);

            return new HttpResponseMessage(HttpStatusCode.Created);
        }