反序列化Json Web API响应-嵌套对象

Question

I am getting a Web API JSON Response via SSIS Script component, that is coming back in the following format:

    {
  "Success": true,
  "Response": {
    "Applications": [
      {
        "App_ID": 1638486,
        "App_Ref": "Test Example",
        "Status": "Complete",
        "Error_Code": null,
        "Error_Message": null,
        "Create_Dt": "2014-05-14 03:09:01.030 +00:00",
        "Modify_Dt": "2014-05-14 03:10:59.757 +00:00",
        "Client_Name": "Silver Chef",
        "Client_Code": "SLVC01",
        "Centrelink_Status": "Receiving_Logons"
      },
      {
        "App_ID": 1637906,
        "App_Ref": "SME Demo",
        "Status": "Complete",
        "Error_Code": null,
        "Error_Message": null,
        "Create_Dt": "2015-10-08 03:07:26.793 +00:00",
        "Modify_Dt": "2015-10-08 03:23:32.833 +00:00",
        "Client_Name": "Silver Chef",
        "Client_Code": "SLVC01",
        "Centrelink_Status": "Receiving_Logons"
      },
      {
        "App_ID": 1585286,
        "App_Ref": "Test",
        "Status": "Receiving_Logons",
        "Error_Code": null,
        "Error_Message": null,
        "Create_Dt": "2015-12-04 03:12:49.617 +00:00",
        "Modify_Dt": "2015-12-04 03:12:49.617 +00:00",
        "Client_Name": "Silver Chef",
        "Client_Code": "SLVC01",
        "Centrelink_Status": "Receiving_Logons"
      }
],
    "Current_Dt": "2016-11-11 01:01:01.743 +00:00",
    "Last_App_Dt": "2016-10-03 22:48:56.397 +00:00",
    "Count": 500,
    "Total": 1870
  }
}

I have declared the following classes:

     public class Application
        {
            public int App_ID { get; set; }
            public string App_Ref { get; set; }
            public string Status { get; set; }
            public int Error_Code { get; set; }
            public string Error_Message { get; set; }
            public DateTime Create_Dt { get; set; }
            public DateTime Modify_Dt { get; set; }
            public string Client_Name { get; set; }
            public string Client_Code { get; set; }
            public int Centrelink_Status { get; set; }

        }
        public class Response
        {
            public List<Application> Applications { get; set; }
            public string Current_Dt { get; set; }
            public string Last_App_Dt { get; set; }
            public int Count { get; set; }
            public int Total { get; set; }
        }

        public class RootObject
        {
            public bool Success { get; set; }
            public Response Response { get; set; }
        }

And this is the method that I am using to get the response. 
private RootObject GetWebServiceResult(string vAPIUrl)
    {

        string vAPIToken = Variables.APIToken;

        //Create Web Request
        HttpWebRequest apireq = (HttpWebRequest)WebRequest.Create(vAPIUrl);
        apireq.ContentType = "application/json";
        apireq.Method = "POST";

        string jsonPostStr = "{\"Settings\": {\"API_Token\": \"" + vAPIToken + "\"}, \"Payload\": {}}";
        byte[] postString = Encoding.UTF8.GetBytes(jsonPostStr);

        apireq.ContentLength = postString.Length;

        Stream jsonStream = apireq.GetRequestStream();

        jsonStream.Write(postString, 0, postString.Length);
        jsonStream.Close();

        // Get Web Response        
        HttpWebResponse apirsp = (HttpWebResponse)apireq.GetResponse();
        RootObject jsonResponse = null;

        Stream jsonRspStream = apirsp.GetResponseStream();
        string apiResponseString = null;

        using (StreamReader reader = new StreamReader(jsonRspStream)) 
        {
            apiResponseString = reader.ReadToEnd(); 
            Console.WriteLine(apiResponseString);
            reader.Close();
        }


        JavaScriptSerializer returnJson = new JavaScriptSerializer();

        //var serialJsonStr = returnJson.Serialize(apiResponseString);
        System.Windows.Forms.MessageBox.Show(apiResponseString);

        jsonResponse = returnJson.Deserialize<RootObject>(apiResponseString);

        return jsonResponse;

    }

My issue is that returnJson.Deserialize(apiResponseString); seems to return null and subsequently makes this fail.

What am i missing? i think i am slightly code blind at this stage.

Thanks in advance

Answer 1

There are a couple of problems with the Application class:

• You have defined Application.Error_Code to be an integer:

   public int Error_Code { get; set; } 

  But in fact, in the JSON, it has a null value:

   "Error_Code": null, 

  Thus Error_Code needs to be a reference type ( string ) or nullable value type ( int? ) depending on what it holds when non-null.

• You have defined Centrelink_Status as an int , but in the JSON it's a string:

   "Centrelink_Status": "Receiving_Logons" 

  Thus it needs to be a string in the data model as well.

Thus your Application class should look like:

public class Application
{
    public int App_ID { get; set; }
    public string App_Ref { get; set; }
    public string Status { get; set; }
    public int? Error_Code { get; set; } // Or string, if not numeric
    public string Error_Message { get; set; }
    public DateTime Create_Dt { get; set; }
    public DateTime Modify_Dt { get; set; }
    public string Client_Name { get; set; }
    public string Client_Code { get; set; }
    public string Centrelink_Status { get; set; }
}

You might also want to make those DateTime properties be nullable if there is a chance for a null value in the JSON.

Incidentally, when I tried to deserialize this JSON, I got an exception thrown, rather than a null value returned:

System.InvalidOperationException was unhandled
  Message="Cannot convert null to a value type."
  Source="System.Web.Extensions"
  StackTrace:

In your actual code, are you catching and swallowing exceptions? That's a bad idea precisely because it tends to hide errors such as this. At the minimum, when debugging you should break whenever any exception is thrown including a first-chance exception - even though this is not the default behavior. See Managing Exceptions with the Debugger . If you find there are certain types of exception that are frequently thrown and are irrelevant to debugging, you can selectively re-ignore them. Had you done so this bug would have been more straightforward to track down. Admittedly, the error is not very explanatory, but it does give a hint. If you were to use json.net to deserialize your JSON, the error message would be much clearer:

Newtonsoft.Json.JsonSerializationException occurred
  Message="Error converting value {null} to type 'System.Int32'. Path 'Response.Applications[0].Error_Code', line 9, position 26."
  Source="Newtonsoft.Json"
  StackTrace:
       at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.EnsureType(JsonReader reader, Object value, CultureInfo culture, JsonContract contract, Type targetType) in C:\Development\Releases\Json\Working\Newtonsoft.Json\Working-Signed\Src\Newtonsoft.Json\Serialization\JsonSerializerInternalReader.cs:line 986
  InnerException: System.InvalidCastException
       Message="Null object cannot be converted to a value type."
       Source="mscorlib"
       StackTrace:
            at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
            at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.EnsureType(JsonReader reader, Object value, CultureInfo culture, JsonContract contract, Type targetType) in C:\Development\Releases\Json\Working\Newtonsoft.Json\Working-Signed\Src\Newtonsoft.Json\Serialization\JsonSerializerInternalReader.cs:line 979
       InnerException: