asp.net unable to cast object of type error

Question

I am getting an error when trying to convert json response from coinmarketcap.com's API.
The error is:

"Unable to cast object of type 'System.Collections.Generic.List`1[System.Object]' to type 'System.Dynamic.ExpandoObject'"

The web client works fine for other API's but for some reason coin object is not populated. Any insight into a fix in either vb.net or c# would be much appreciated.

    Dim dt_results As DataTable, dr As DataRow, url As String = String.Empty, json As Object = Nothing, iCount As Integer = 0

    'temporarily store results
    dt_results = New DataTable
    dt_results.Columns.Add("name", GetType(String))
    dt_results.Columns.Add("symbol", GetType(String))
    dt_results.Columns.Add("price_usd", GetType(Double))

    Try
        url = "https://api.coinmarketcap.com/v1/ticker/?convert=usd&limit=10"
        Dim theurl As New Uri(url)

        Using webClient = New System.Net.WebClient()

            json = webClient.DownloadString(theurl)

            'create json object
            Dim converter = New ExpandoObjectConverter()
            Dim coin As Object = JsonConvert.DeserializeObject(Of ExpandoObject)(json, converter)

            For Each item In coin

                Dim name As String = coin(iCount).name
                Dim symbol As String = coin(iCount).symbol
                Dim price_usd As Double = coin(iCount).price_usd
                dr = dt_results.NewRow()
                dr("name") = name
                dr("symbol") = symbol
                dr("price_usd") = price_usd
                dt_results.Rows.Add(dr)
                iCount = iCount + 1
            Next
        End Using

    Catch ex As Exception
        Dim ts As String = ex.Message
        json = "1"
    End Try

Working solution...

        Dim d As JArray = JArray.Parse(json)

            For i As Integer = 0 To d.Count
                Dim name As String = d(i).Item("name")
                Dim symbol As String = d(i).Item("symbol")
                Dim price_usd As Double = CDbl(d(i).Item("price_usd"))
                dr = dt_results.NewRow()
                dr("name") = name
                dr("symbol") = symbol
                dr("price_usd") = price_usd
                dt_results.Rows.Add(dr)
                iCount = iCount + 1
            Next

mrkg · Accepted Answer · 2017-12-06 20:01:31Z

1

Haven't written in VB.NET for a very long while, but I have a solution for you in C#. If you do not have a clearly defined object model, I would recommend using Json.Net, parsing it as a dynamic object. We're getting and array from the API, so we need to tell Json.Net that we're expecting it to parse it as such. Below is a working solution for me (albeit in C#)

using (WebClient cli = new WebClient())
        {
            string result = cli.DownloadString("https://api.coinmarketcap.com/v1/ticker/?convert=usd&limit=10");
            dynamic arrayFromApi = JArray.Parse(result);

            // Use however - presumably loop through items
            string s = arrayFromApi[0].name;
        }

answered Dec 6, 2017 at 20:01

mrkg

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1 Comment

mrkg Over a year ago

Glad I could help!

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