Question

Trying to deserialise the following json returned from a web source:

{
    "cards": [
    {
        "high": "8.27", 
        "volume": 5, 
        "percent_change": "0.00", 
        "name": "Wurmcoil engine", 
        "url": "http://blacklotusproject.com/cards/Scars+of+Mirrodin/Wurmcoil+Engine/", 
        "price": "6.81", 
        "set_code": "SOM", 
        "average": "5.67", 
        "change": "0.00", 
        "low": "1.12"}], 
        "currency": "USD"
    }

I am using json.net with visual basic, new to both of them, especially the object oriented portions of vb. I would just like to extract the ‘price’ variable.
I have set up a class as such:

Public Class Card
    Public high As String
    Public volume As String
    Public percent_change As String
    Public name As String
    Public url As String
    Public price As String
    Public set_code As String
    Public average As String
    Public change As String
    Public low As String
End Class

The code I am currently using is:

Public Sub parse_json(url As String)
    Dim blp_json As String = ""
    Dim wClient As New WebClient
    wClient.Proxy = System.Net.HttpWebRequest.DefaultWebProxy
    blp_json = wClient.DownloadString(url)
    MessageBox.Show(blp_json)
    Dim card_1 = Newtonsoft.Json.JsonConvert.DeserializeObject(Of Card)(blp_json)
    PriceTextBox.Text = card_1.price
    TextBox1.AppendText(card_1.ToString)
    TextBox1.AppendText(blp_json)

End Sub

Just trying many different things to get a hang of it, not really sure what I am doing. I presume my Card class is incorrect as ‘price’ appears to be nested in cards:[{…}]

I don’t really know about deserialising json at all, and much less about how to do it /properly/ in vb.

Answer 1

I use the System.Web.Script.Serialization.JavaScriptSerializer for JSON deserialization. Your example is slightly complicated by the fact that cards contains an array of JSON objects. This is indicated by the “[” and “]”. This example code will show you how to process it regardless of whether cards is an array or not. You may wish to ignore the Else if you are sure that there will always be an array in cards

Make sure that you have included a reference to System.Web.Extensions in your project and…

Imports System.Web.Script.Serialization

and then…

Dim MySerializer As JavaScriptSerializer = New JavaScriptSerializer()
Dim dictResult As Dictionary(Of String, Object) = MySerializer.Deserialize(Of Dictionary(Of String, Object))(blp_json)

Dim dictCard As Dictionary(Of String, Object)

If dictResult.ContainsKey("cards") Then

    If TypeOf dictResult("cards") Is ArrayList Then

        Dim arrResult As ArrayList = DirectCast(dictResult("cards"), ArrayList)

        For Each arrCardRecord In arrResult

            dictCard = DirectCast(arrCardRecord, Dictionary(Of String, Object))

            If dictCard.ContainsKey("price") Then Console.WriteLine(dictCard("price"))

        Next
    Else
        dictCard = DirectCast(dictResult("cards"), Dictionary(Of String, Object))
        If dictCard.ContainsKey("price") Then Console.WriteLine(dictCard("price"))
    End If

End If