This could be a reason on syntax, but not on api statements. So correct the syntax if needed.
Just do a quick break, take a breath and continue after this fresh and concentrated. I am sure you will get it working
give a try on:
xDoc.Root.Descendants(xnsTempuri + "Result").First().Descendants(xnsVersion + "IDNumber").First().Value;
OR
xDoc.Root.Descendants(xnsTempuri + "Result").ElementAt(0).Descendants(xnsVersion + "Code").ElementAt(0).Value;