Generating XML file using dotnet

using System.Xml;
using System.IO;
using System.Web.Script.Serialization;

private void CreateANDSaveXML()
    {      

        string xmlString = "<?xml version='1.0' encoding='UTF-8'?>";
        xmlString += "<Details>";
        xmlString += "<FirstName>" + "Test First Name" + "</FirstName>";
        xmlString += "<LastName>" + "Test Last Name" + "</LastName>";
        xmlString += "<Gender>" + "Test Gender" + "</Gender>";       
        xmlString += "</Details>";

        XmlDocument doc = new XmlDocument();
        doc.LoadXml(xmlString.Replace("&nbsp;", ""));
        if (Session["USER"] != null)
        {
            if (File.Exists(Server.MapPath(".\\FolderName\\XML\\") + "FileName_" + Session["USER"].ToString() + ".xml"))
            {
                File.Delete(Server.MapPath(".\\FolderName\\XML\\") + "FileName_" + Session["USER"].ToString() + ".xml");
            }
            doc.Save(Server.MapPath(".\\FolderName\\XML\\") + "FileName_" + Session["USER"].ToString() + ".xml");
        }
       
    }