ASPC C# - (500) Internal Server Error when posting JSON - json

Question

I am trying to POST some JSON data to a remote server and read JSON response back from the remote server. My code is jumping in to the catch exception block with the error ex = {"The remote server returned an error: (500) Internal Server Error."} when it gets to this line near the bottom:
var httpResponse = (HttpWebResponse)request.GetResponse();
I have read a few posts on this forum and tried the code other suggest but I don't understand/can't get it to work.
Please can you help me understand what I am doing wrong? You can see my previous attempt which is commented out near the bottom of the code block.
using System;
using System.Text;
using System.Net;
// include
using System.IO;
using System.Web.UI.WebControls;
using HobbsDPDJSONLibrary;
using System.Web;
using Newtonsoft.Json;
namespace DPDAPILibrary
{
public class DPD_API
{
#region public class variables
private static string dpdapiun = "xxx";
private static string dpdapipw = "xxx";
private static string dpdAccountNumber = "xxx";
private static string dpdapihost = "api.dpd.co.uk";
private static string dpdapiinserttestshipment = "https://api.dpd.co.uk/shipping/shipment?test=true";
private static string dpdapiinsertshipment = "https://api.dpd.co.uk/shipping/shipment";
#endregion
/// <summary>
/// Send consignment data to the DPD API to create a shipment and return a consignment number (if successful).
/// </summary>
/// <param name="geoClientData"></param>
/// <param name="JSONData"></param>
/// <returns></returns>
public Boolean insertShipment(string geoSession, bool test, out string JSONdata)
{
try
{
// default output values
JSONdata = "";
bool returnValue = false;
#region create new insert shipment object
// a large block of code here that serialises a class into JSON, this bit works so I have omitted it to reduce the code I post on the forum
#endregion
string InsertShipmentData = JsonConvert.SerializeObject(NewShipmentObject);
// convert the 'insert shipment' JSON data to byte array for posting
//byte[] postBytes = Encoding.UTF8.GetBytes(InsertShipmentData);
// set the target uri for the insert shipment request (defaults to test, or switch to live as per input parameter)
Uri targetURI = new Uri(dpdapiinserttestshipment);
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(dpdapiinserttestshipment);
if(!test)
{
targetURI = new Uri(dpdapiinsertshipment);
request = (HttpWebRequest)WebRequest.Create(dpdapiinsertshipment);
}
// add headers to the web request for inserting a new shipment
request.Host = dpdapihost;
request.ContentType = "application/json";
request.Accept = "application/json";
request.Method = "POST";
request.Timeout = 30000;
request.KeepAlive = true;
request.AllowAutoRedirect = false;
request.Headers["GEOClient"] = "thirdparty/" + dpdAccountNumber;
request.Headers["GeoSession"] = geoSession;
using (var streamWriter = new StreamWriter(request.GetRequestStream()))
{
streamWriter.Write(InsertShipmentData);
}
var httpResponse = (HttpWebResponse)request.GetResponse();
using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
{
var result = streamReader.ReadToEnd();
}
//// run the request and read the response header
//using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
//{
// using (var sr = new StreamReader(response.GetResponseStream()))
// {
// JSONdata = Convert.ToString(sr.ReadToEnd());
// }
// // check if OK (status 200) returned
// if (response.StatusCode.ToString() == "OK")
// {
// returnValue = true;
// }
// else
// {
// returnValue = false;
// }
// return returnValue;
//}
return returnValue;
}
catch (Exception ex)
{
JSONdata = Convert.ToString(ex);
return false;
}
}
}
}