public Car Add(Car car) {

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

            return car;
        }
        // POST /api/cars
        public HttpResponseMessage PostCar(Car car) {

            var createdCar = _carsCtx.Add(car);
            var response = Request.CreateResponse(HttpStatusCode.Created, createdCar);
            response.Headers.Location = new Uri(
                Url.Link("DefaultApiRoute", new { id = createdCar.Id }));

            return response;
        }
        public bool TryUpdate(Car car) {

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

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

            return false;
        }