public static async Task <ObservableCollection <Rental> > GetCartAsync()
        {
            using (HttpResponseMessage response = await ShoppingCartController.GetCart())
            {
                int    status = (int)response.StatusCode;
                string body   = await response.Content.ReadAsStringAsync();

                if (status == 200)
                {
                    ObservableCollection <Rental> jsonBody = JsonConvert.DeserializeObject <ObservableCollection <Rental> >(body);
                    return(jsonBody);
                }
                else if (status == 400)
                {
                    throw new ArgumentException(body);
                }
                else if (status == 401)
                {
                    throw new UnauthenticatedException();
                }
                else if (status == 404)
                {
                    return(new ObservableCollection <Rental>());
                }
                else if (status == 500)
                {
                    throw new ServerException();
                }
                else
                {
                    throw new Exception("An error occurred");
                }
            }
        }