使用Newtonsoft将JSON从文件解析到类时出错

Question

我正在尝试读取文件数据，然后将其传递给类进行进一步处理。正如我所看到的，obj正在接受数据，但随后我出现错误：

Value cannot be null.
Parameter name: source

这是代码：

var url = HttpContext.Current.Server.MapPath(@"~/App_Data/" + username + "/prices.json");
            PricesViewModel.DepartureFlightsData dfd = null;
            if (File.Exists(url))
            {
                JObject obj = JObject.Parse(File.ReadAllText(url));
                dfd = (from a in obj["PriceItems"] 
                      where a["CityFromID"].Value<string>().Contains(cityfromcode) && a["CityToID"].Value<string>().Contains(citytocode) && a["AirlineCode"].Value<string>().Contains(airlinecode)
                      select new PricesViewModel.DepartureFlightsData //this line throw exception
                {
                    DepartureCity = a["CityFromName"].Value<string>(),
                    DepartureAirport = a["AirportFromCode"].Value<string>(),
                    DepartureDate = depDate,
                    DepartureTime = a["DepartureTime"].Value<string>(),
                    DepartureAirline = a["AirlineCode"].Value<string>(),
                    DepartureFlight = a["DepartureFlight"].Value<string>(),
                    DepartureFlightId = a["FlightId"].Value<string>(),
                    Price = a["Price"].Value<int>(),
                    Currency = a["Currency"].Value<string>()
                })
                .FirstOrDefault();

这就是类结构（DepartmentFlightData是PricesViewModel类的一部分）：

  public class DepartureFlightsData
        {
            public string DepartureCity { get; set; }
            public string DepartureAirport { get; set; }
            public DateTime DepartureDate { get; set; }
            public string DepartureTime { get; set; }
            public string DepartureAirline { get; set; }
            public string DepartureFlight { get; set; }
            public string DepartureFlightId { get; set; }
            public int Price { get; set; }
            public string Currency { get; set; }
        }

好的，JSON在这里：

{
  "PriceItems": [
    {
      "FlightID": "565",
      "FlightNumber": "731",
      "CityFromID": "1",
      "CityFromCode": "MOW",
      "CityToID": "19",
      "CityToCode": "BCN",
      "AirportFromCode": "DME",
      "AirportFromName": null,
      "AirportToCode": "BCN",
      "AirportToName": null,
      "DepartureDate": "20150227",
      "DepartureTime": "17:00",
      "ArrivalDate": "20150227",
      "ArrivalTime": "21:00",
      "Price": "350",
      "Currency": "EU",
      "AirServiceID": "89",
      "AirCraft": "Boeing",
      "AirlineName": "TRANSAERO",
      "AirlineCode": "UN",
      "PackageID": "232",
      "CityFromName": "Москва",
      "CityToName": "Барселона",
      "TariffName": "Эконом",
      "FinalPrice": 700,
      "Quota": "Есть"
    },
    {
      "FlightID": "563",
      "FlightNumber": "2639",
      "CityFromID": "1",
      "CityFromCode": "MOW",
      "CityToID": "19",
      "CityToCode": "BCN",
      "AirportFromCode": "SVO",
      "AirportFromName": null,
      "AirportToCode": "BCN",
      "AirportToName": null,
      "DepartureDate": "20150227",
      "DepartureTime": "11:50",
      "ArrivalDate": "20150227",
      "ArrivalTime": "19:15",
      "Price": "350",
      "Currency": "EU",
      "AirServiceID": "89",
      "AirCraft": "Boeing",
      "AirlineName": "Aeroflot-Russian International AirLines",
      "AirlineCode": "SU",
      "PackageID": "232",
      "CityFromName": "Москва",
      "CityToName": "Барселона",
      "TariffName": "Эконом",
      "FinalPrice": 700,
      "Quota": "Есть"
    },
...
}

Answer 1

试试这个

    DepartureFlightsData foo()
    {
         String jsonText = File.ReadAllText(url);
         if(String.IsNullOrEmpty(jsonText)) throw new Exception("JSON data not found");
         var flights = Newtonsoft.Json.JsonConvert
                    .DeserializeObject<IEnumerable<DepartureFlightsData>>(jsonText);
         return flights.First(); //will throw exception if it's empty
    }