public City Create(City newCity)
        {

            MockDatabase.Cities.Add(newCity);

            return newCity;
        }

        public bool Delete(int id)
        {
            var city = MockDatabase.Cities.FirstOrDefault(c => c.Id == id);

            return MockDatabase.Cities.Remove(city);
        }

        public City Get(int id)
        {
            var city = MockDatabase.Cities.FirstOrDefault(c => c.Id == id);

            if (city == null)
            {
                throw new ArgumentNullException();
            }

            return city;
        }

        public List<City> GetAll()
        {
            return MockDatabase.Cities;
        }

        public City Update(int id, string newName, int newCountry)
        {

            var city = MockDatabase.Cities.FirstOrDefault(c => c.Id == id);

            if (city == null)
            {
                throw new ArgumentNullException();
            }

            city.Name = newName;
            city.CountryId = newCountry;
            return city;
        }