public IReadOnlyList <AirTariff> GetAirTariffs()
        {
            if (Tariffs == null)
            {
                return(new List <AirTariff>());
            }

            return(Tariffs.OfType <AirTariff>().ToList());
        }
        public IEnumerable <AirTariff> GetAirTariffs()
        {
            if (Tariffs == null)
            {
                return(Enumerable.Empty <AirTariff>());
            }

            return(Tariffs.OfType <AirTariff>());
        }