Webservice return error 500

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C#

webservice

Dear all,
I try to make simple desktop application that calling some webservice. I make webservice using wso2 esb. But when I calling the service through my application it return error 500. Here is my class that calling the web service

private void button3_Click(object sender, EventArgs e)
{
    try
    {
        ChangeUrl("https://192.168.24.129:9443/services/rate_DataService/");
        ExampleWebMethod(txtFrom.Text, txtTo.Text, 12456);

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}
public static void ChangeUrl(string webserviceEndpoint)
{
    ExampleAPI = new WebService(webserviceEndpoint);
}

public static string ExampleWebMethod(string from, string to, decimal number)
{
    ExampleAPI.PreInvoke();

    ExampleAPI.AddParameter("param0", from);
    ExampleAPI.AddParameter("param1", to);
    ExampleAPI.AddParameter("param2", number.ToString());

    try
    {
        ExampleAPI.Invoke("ExampleWebMethod");
    }
    finally { ExampleAPI.PosInvoke(); }

    return ExampleAPI.ResultString;
}

and this is the Webservice class that I create

public void Invoke(string methodName)
{
    Invoke(methodName, true);
}

private void Invoke(string methodName, bool encode)
{
    AssertCanInvoke(methodName);
    string soapStr =
        @"<?xml version=""1.0"" encoding=""utf-8""?>
        <soap:Envelope xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance""
           xmlns:xsd=""http://www.w3.org/2001/XMLSchema""
           xmlns:soap=""http://schemas.xmlsoap.org/soap/envelope/"">
          <soap:Body>
            <{0} xmlns=""http://tempuri.org/"">
              {1}
            </{0}>
          </soap:Body>
        </soap:Envelope>";

    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(Url);
    req.ServerCertificateValidationCallback = ((sender, certificate, chain, sslPolicyErrors) => true);

    req.Headers.Add("SOAPAction", "\"http://tempuri.org/" + methodName + "\"");
    req.ContentType = "text/xml;charset=\"utf-8\"";
    req.Accept = "text/xml";
    req.Method = "POST";


    using (Stream stm = req.GetRequestStream())
    {
        string postValues = "";
        foreach (var param in Params)
        {
            if (encode) postValues += string.Format("<{0}>{1}</{0}>", HttpUtility.HtmlEncode(param.Key), HttpUtility.HtmlEncode(param.Value));
            else postValues += string.Format("<{0}>{1}</{0}>", param.Key, param.Value);
        }

        soapStr = string.Format(soapStr, methodName, postValues);
        using (StreamWriter stmw = new StreamWriter(stm))
        {
            stmw.Write(soapStr);
        }
    }

    using (StreamReader responseReader = new StreamReader(req.GetResponse().GetResponseStream()))
    {
        string result = responseReader.ReadToEnd();
        ResponseSOAP = XDocument.Parse(Utils.UnescapeString(result));
        ExtractResult(methodName);
    }
}

What I have tried:

I already try googling it and some said i need to setting the jsonSerialization size but since I using wso2 I cannot find where the web config located.

Is there anyone who know how to solved this problem

Thank you