對象引用不設置到對象的實例

Question

我想從以下網址https://www.gov.uk/api/foreign-travel-advice.json

我已經使用http://json2csharp.com/提取類獲取數據，但我不能讓我之後的數據。

任何人誰比我更有經驗請看看，讓我知道我要去哪裏錯了。

這是投擲空全球化志願服務青年

var data = (from j in o["countries"].Children() 
         select new Country 
         { 
           Id  = (string)j["id"], 
           WebUrl = (string)j["name"] 
         }); 

全碼

namespace Web.Domain.FCO.Countries 
{ 
    public class Details 
    { 
     public string Description { get; set; } 
     public object ShortDescription { get; set; } 
     public string Type { get; set; } 
    } 

    public class ContentWithTag 
    { 
     public string Id { get; set; } 
     public string WebUrl { get; set; } 
    } 

    public class Details2 
    { 
     public string Description { get; set; } 
     public object ShortDescription { get; set; } 
     public string Type { get; set; } 
    } 

    public class ContentWithTag2 
    { 
     public string Id { get; set; } 
     public string WebUrl { get; set; } 
    } 

    public class Parent 
    { 
     public string Id { get; set; } 
     public object WebUrl { get; set; } 
     public string Title { get; set; } 
     public Details2 Details { get; set; } 
     public ContentWithTag2 ContentWithTag { get; set; } 
     public object parent { get; set; } 
    } 

    public class Tag 
    { 
     public string Id { get; set; } 
     public object WebUrl { get; set; } 
     public string Title { get; set; } 
     public Details Details { get; set; } 
     public ContentWithTag ContentWithTag { get; set; } 
     public Parent Parent { get; set; } 
    } 

    public class Related 
    { 
     public string Id { get; set; } 
     public string WebUrl { get; set; } 
     public string Title { get; set; } 
     public string Format { get; set; } 
     public string UpdatedAt { get; set; } 
     public string Group { get; set; } 
    } 

    public class Country 
    { 
     public string Id { get; set; } 
     public string Name { get; set; } 
     public string Identifier { get; set; } 
     public string WebUrl { get; set; } 
     public string UpdatedAt { get; set; } 
     public string ChangeDescription { get; set; } 
     public List<object> Synonyms { get; set; } 
    } 

    public class Details3 
    { 
     public string NeedId { get; set; } 
     public bool BusinessProposition { get; set; } 
     public string Description { get; set; } 
     public string Language { get; set; } 
     public bool NeedExtendedFont { get; set; } 
     public List<Country> Countries { get; set; } 
    } 

    public class ResponseInfo 
    { 
     public string Status { get; set; } 
    } 

    public class FcoFullDetails 
    { 
     public string Id { get; set; } 
     public string WebUrl { get; set; } 
     public string Title { get; set; } 
     public string Format { get; set; } 
     public string UpdatedAt { get; set; } 
     public List<Tag> Tags { get; set; } 
     public List<Related> Related { get; set; } 
     public Details3 Details { get; set; } 
     public List<object> RelatedExternalLinks { get; set; } 
     public ResponseInfo ResponseInfo { get; set; } 
    } 

    public class IdAndUrl 
    { 
     public string Id { get; set; } 
     public string WebUrl { get; set; } 
    } 

    public class DisplayFcoCountryList : IDisplayFcoCountryList 
    { 
     public IEnumerable<Country> ListOfCountries() 
     { 
      var doc = FcoCountriesAdvice.GetFcoCountryLinks().Result; 
      // var jd = new [] {JsonConvert.DeserializeObject(doc)}; 
      var jd = JsonConvert.DeserializeObject(doc); 
      JObject o = JObject.Parse(jd.ToString()); 

      //var data = (from j in o["countries"] 
         var data = (from j in o["countries"].Children() 
         select new Country 
         { 
           Id  = (string)j["id"], 
           WebUrl = (string)j["name"] 
         }); 
      return data; 
     } 
    } 
    public static class FcoCountriesAdvice 
    { 
     private static readonly string Key = ConfigurationManager.AppSettings["FCO_AdviceURL"]; 

     public static async Task<string> GetFcoCountryLinks() 
     { 
      using (var ta = new HttpClient()) 
      { 
       const string cacheDataTravelAdvice = "FCOCountryLinks"; 

       ObjectCache travelCache = MemoryCache.Default; 
       var objectInCache = travelCache.Get(cacheDataTravelAdvice) as string; 

       if (objectInCache != null) 
       { 
        return objectInCache; 
       } 

       var url = await ta.GetStringAsync(string.Format("{0}", Key)).ConfigureAwait(false); 

       var policy = new CacheItemPolicy { AbsoluteExpiration = DateTime.Now.AddMinutes(15) }; 

       travelCache.Add(cacheDataTravelAdvice, url, policy); 

       return url; 
      } 
     } 
    } 
} 

Answer 1

一個nullref例外，同時創造一個LINQ聲明將最有可能意味着部分代碼：o["countries"].Children()返回NULL，因此不能被迭代。嘗試使用一些簡單的語句對其進行調試時會發生什麼情況：

Console.WriteLine（o [「countries」]）;

Console.WriteLine（o [「countries」]。Children（））; （）;（）; Children（）。Count（））;（）;}};

Answer 2

嘗試使用json解串器如JSON.net。這確實所有的JSON-C＃轉換爲你：

//make request using the HttpClient class 
    var client = new HttpClient{BaseAddress = new Uri("https://www.gov.uk/api/")}; 
    var response = client.GetAsync("foreign-travel-advice.json").Result; 

    //use Json.Net to convert json to c# classes using generics 
    var content = JsonConvert.DeserializeObject<RootObject> (response.Content.ReadAsStringAsync().Result); 

Answer 3

阿萊西奧回答解決了這一問題，參觀他提供的鏈接後，我注意到，我應該一直在使用VAR數據=（從j鄰[「細節「）[」countries「]。兒童（）不是var data =（來自j中的o [」國家「]兒童（）。問題現在已修復