AddNewCity.cs

private bool AddNewCity(string cityName, string countryIso)
{
    var city = CountriesCitiesDbService.GetNewCity();
    var europeanCountries = CountriesCitiesDbService.GetAllEuropeanCountries();
    var country = europeanCountries.FirstOrDefault(ec => ec.IsoCode.Equals(countryIso));

    if (country == null)
        throw new ArgumentException($"Country with ISO Code {countryIso} does not exist!");

    city.Name = cityName;
    city.Country = country;

    CountriesCitiesDbService.SaveCity(city);
    return true;

}