Exception while parsing Json

Question

Hi, I'm parsin a Json API

which is basically an API from one crypto coin of a crypto exchange. Now the crypto coin is under maintenance, so the trading is closed and the Json is changed.
The JSON when the trading coin is open is:

{"error":null,"result":{"KIBAUSDT":{"limit":100,"offset":0,"total":1,"records":[{"user":2996434,"id":82651361103,"market":"KIBAUSDT","freeze":"0.001","fee_stock":"","source":"web","type":1,"side":2,"ctime":1645022087.403207,"mtime":1645022087.403207,"price":"0.00001","amount":"100","taker_fee":"0.0035","maker_fee":"-0.003","left":"100","deal_stock":"0","deal_money":"0","deal_fee":"0","alt_fee":"0","deal_fee_alt":"0","status":0}]}},"id":305636871}

While now that the trading is in maintenance, the Json is:

{"error":{"code":6,"message":"RET_TRADE_NOT_OPENKIBAUSDT"},"result":null,"id":0}

The code I was using in order to parse the JSON when the trading is open is:

Option Strict On
Imports System
Imports Newtonsoft.Json
Imports Newtonsoft.Json.Converters
Imports System.Runtime.CompilerServices

Namespace OrdiniAperti
    Partial Public Class Openorders
        
        Public Property [Error] As Object
        
        Public Property Result As Result
        
        Public Property Id As Double
    End Class

    Partial Public Class Result
        
        Public Property Kibausdt As Kibausdt
    End Class

    Partial Public Class Kibausdt
        
        Public Property Limit As Double
        
        Public Property Offset As Double
        
        Public Property Total As Double
        
        Public Property Records As Record()
    End Class

    Partial Public Class Record
        
        Public Property User As Double
        
        Public Property Id As Double
        
        Public Property Market As String
        
        Public Property Freeze As String
        
        Public Property FeeStock As String
        
        Public Property Source As String
        
        Public Property Type As Double
        
        Public Property Side As Double
        
        Public Property Ctime As Double
        
        Public Property Mtime As Double
        
        Public Property Price As String
        
        
        Public Property Amount As Double
        
        Public Property TakerFee As String
        
        Public Property MakerFee As String
        
        
        Public Property Left As Double
        
        
        Public Property DealStock As Double
        
        
        Public Property DealMoney As Double
        
        
        Public Property DealFee As Double
        
        
        Public Property AltFee As Double
        
        
        Public Property DealFeeAlt As Double
        
        Public Property Status As Double
    End Class

    Partial Public Class Openorders
        Public Shared Function FromJson(ByVal json As String) As Openorders
            Return JsonConvert.DeserializeObject(Of Openorders)(json, Settings)
        End Function
    End Class

    Public Module Serialize
        
        Public Function ToJson(ByVal self As Openorders) As String
            Return JsonConvert.SerializeObject(self, Settings)
        End Function
    End Module

    Friend Module Converter
        Public ReadOnly Settings As JsonSerializerSettings = New JsonSerializerSettings With {
            .MetadataPropertyHandling = MetadataPropertyHandling.Ignore,
            .DateParseHandling = DateParseHandling.None
}
        'System.Threading.Thread.CurrentThread.CurrentCulture = System.Globalization.CultureInfo.InvariantCulture

    End Module

    Friend Class ParseStringConverter
        Inherits JsonConverter

        Public Overrides Function CanConvert(ByVal t As Type) As Boolean
            Return t Is GetType(Double) OrElse t Is GetType(Double?)
        End Function

        Public Overrides Function ReadJson(ByVal reader As JsonReader, ByVal t As Type, ByVal existingValue As Object, ByVal serializer As JsonSerializer) As Object
            If reader.TokenType = JsonToken.Null Then Return Nothing
            Dim value = serializer.Deserialize(Of String)(reader)
            Dim l As Double

            If Double.TryParse(value, l) Then
                Return l
            End If

            Throw New Exception("Cannot unmarshal type Double")
        End Function

        Public Overrides Sub WriteJson(ByVal writer As JsonWriter, ByVal untypedValue As Object, ByVal serializer As JsonSerializer)
            If untypedValue Is Nothing Then
                serializer.Serialize(writer, Nothing)
                Return
            End If

            Dim value = CLng(untypedValue)
            serializer.Serialize(writer, value.ToString())
            Return
        End Sub

        Public Shared ReadOnly Singleton As ParseStringConverter = New ParseStringConverter()
    End Class
End Namespace

and in a button:

 Dim json = Await wc1.DownloadStringTaskAsync(TextBox1.Text)
                'deserialize the json
                Dim rootObj = Openorders.FromJson(json)
                'navigate to Kibausdt ( when trading is open)
                Dim kibausdt = rootObj.Result.Kibausdt
                'navigate to Id ( when trade is closed)
                Dim opentrading = rootObj.Id

                If opentrading = 0 Then
                    RichTextBox3.Text = "trading not open"
                Else
                    If kibausdt.Total = 0 Then
                        RichTextBox3.Text = "0 opened orders"

                    Else
                        RichTextBox3.Clear()
                        'loop through records
                        For Each record In kibausdt.Records
                            Dim side As String = record.Side.ToString()
ecc

So as you can see from the code in the button, I'm trying to navigate to Id because id become to 0 when the trading is not open. But unfortunately I'm getting an error at debug, the app goes in break mode and it thrown the following error : System.Reflection.TargetInvocationException: 'exception has been thrown by the target of an invocation
How it is possible? I'm navigate correctly to the Id, so I'm excpeting it to work..
Any idea?
Thanks

Answer

Hi @Mattia Fanti ,
When the trading is not open, Result is null.
So Dim kibausdt = rootObj.Result.Kibausdt will throw an exception.
You can first judge whether the trading is open by error or id, and then perform other operations.
Best Regards.
Jiachen Li

----------

If the answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Answer

Here same logic applied to part of your code, attention to logic,
Sounds obvious, but you can't ask for a children property when parent does not exist, as I know there is no other way other than follow class tree structure and check for null before go further
Just in case check for "error" or "id" can be considered a personal preference, personally I preffer deal with "error" it can give some additional info you might need in future

Dim json = Await wc1.DownloadStringTaskAsync(TextBox1.Text)
    Dim MyData As Openorders = Openorders.FromJson(json)
    If MyData.error IsNot Nothing Then
      RichTextBox3.Text = "Oops, there is an error."
      If MyData.id = 0 Then
        RichTextBox3.Text = RichTextBox3.Text & vbCrLf & "and trading not open"
      End If
    Else
      If MyData.result IsNot Nothing Then
        If MyData.result.KIBAUSDT IsNot Nothing Then
          If MyData.result.KIBAUSDT.total = 0 Then
            RichTextBox3.Text = "0 opened orders"
          Else
            RichTextBox3.Clear()
            For Each record In MyData.result.KIBAUSDT.records
              Dim side As String = record.side.ToString()
            Next
          End If
        Else
          RichTextBox3.Text = "there is no KIBAUSDT"
        End If
      Else
        RichTextBox3.Text = "there is no result"
      End If
    End If

Share via

Exception while parsing Json

2 answers