JSONシリアル化エラーをデバッグする方法は？

Question

私は一貫性がなく、頻繁に発生した次の例外があります。

System.Exception: Serialization error - One or more errors occurred., Unexpected character encountered while parsing value: <. Path '', line 0, position 0., System.AggregateException: One or more errors occurred. ---> Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0. 
    at Newtonsoft.Json.JsonTextReader.ParseValue() 
    at Newtonsoft.Json.JsonTextReader.Read() 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.ReadForType(JsonReader reader, JsonContract contract, Boolean hasConverter) 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent) 
    at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType) 
    at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings) 
    at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings) 
    at Microsoft.AspNet.SignalR.Client.Transports.TransportHelper.b__1(String raw) 
    at Microsoft.AspNet.SignalR.TaskAsyncHelper.<>c__DisplayClass19`2.b__17(Task`1 t) 
    at Microsoft.AspNet.SignalR.TaskAsyncHelper.TaskRunners`2.<>c__DisplayClass42.b__41(Task`1 t) 
    --- End of inner exception stack trace --- 
    at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions) 
    at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken) 
    **at Project.Services.NotificationService.SignalObjectToUser(Object data, String username, Boolean isTypingNotification) 
---> (Inner Exception #0) Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.** 
    at Newtonsoft.Json.JsonTextReader.ParseValue() 
    at Newtonsoft.Json.JsonTextReader.Read() 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.ReadForType(JsonReader reader, JsonContract contract, Boolean hasConverter) 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent) 
    at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType) 
    at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings) 
    at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings) 
    at Microsoft.AspNet.SignalR.Client.Transports.TransportHelper.b__1(String raw) 
    at Microsoft.AspNet.SignalR.TaskAsyncHelper.<>c__DisplayClass19`2.b__17(Task`1 t) 

重要なビット：そこ

**at Project.Services.NotificationService.SignalObjectToUser(Object data, String username, Boolean isTypingNotification) 
---> (Inner Exception #0) Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.** 

マイコード：

private void SignalObjectToUser(object data, string username, bool isTypingNotification = false) { 
     try { 
      _hubConnection = new HubConnection(_baseUrl); 
      _hubConnection.CookieContainer = new CookieContainer(); 
      _hubConnection.CookieContainer.Add(_cookie); 
      _hubProxy = _hubConnection.CreateHubProxy("appHub"); 
      _hubConnection.Start().Wait(); 
      string serialized = null; 
      try { 
       List<string> errors = new List<string>(); 
       serialized = JsonConvert.SerializeObject(data, new JsonSerializerSettings() { 
        Error = delegate(object sender, ErrorEventArgs args) { 
         errors.Add(args.ErrorContext.Error.Message); 
         args.ErrorContext.Handled = true; 
         Elmah.ErrorSignal.FromCurrentContext().Raise(new Exception("Delegated serialization error - " + args.ErrorContext.Error.Message)); 
        } 
       }); 
       _hubProxy.Invoke((isTypingNotification ? "SendTypingNotification" : "SendClientNotification"), serialized, username).Wait(); 
      } 
      catch (Exception exception) { 
       Elmah.ErrorSignal.FromCurrentContext().Raise(new Exception("Serialization error - " + exception.Message + ", " + exception.InnerException.Message + ", " + exception.ToString())); 
       throw new Exception("Messaging Error"); 
      } 
     } 
     catch (Exception e) { 
      Elmah.ErrorSignal.FromCurrentContext().Raise(new Exception("Serialization error - " + e.Message + ", " + e.InnerException.Message + ", " + e.ToString())); 
      throw new Exception("Messaging Error"); 
     } 
    } 

を私は何を把握しようとしているしかし、正確にがこの問題を引き起こしています。私はどのようなキー/値のペアが例外を生成しているのかわかりません。ここではどのようにもっと詳細な情報を集めることができますか？

Answer 1

エンドポイント（_baseUrl）がJSONを返すと思われますが、実際にはXMLまたはHTMLが返されるようです。これは、_baseUrlを要求するときに、内部サーバーエラー、不良リクエスト、見つからなかったり類似している場合に発生します。ブラウザ、PostmanなどでURLを要求しようとすると、ステータスコードとともにHTML応答が表示されることがあります。