Dynamic creation of elements in XML via C#.net

string sFilename = xmlpath;
            XmlDocument xmlDoc = new XmlDocument();
            String XmlSpecification = "<?xml version=\"1.0\" encoding=\"utf-8\" ?>";
            XmlSpecification = XmlSpecification + Environment.NewLine + "<Testing></Testing>";
            File.AppendAllText(xmlpath, XmlSpecification);
            xmlDoc.Load(sFilename);
            XmlElement elmXML = xmlDoc.CreateElement("Configuration");
            string strConfiguration = String.Empty;
            for (int index = 0; index < 3; index++)
            {
                string targetTextBox = "Textbox" + index;
                //Try to find the textbox
                int textBoxIndex = gb.Controls.IndexOfKey(targetTextBox);
                if (textBoxIndex != -1)
                {
                    TextBox foundTextBox = (TextBox)gb.Controls[textBoxIndex];
                    Userprefrenece.txt1 = foundTextBox.Text;
                }
                String str = "port" + index.ToString();
                strConfiguration = strConfiguration + Environment.NewLine + @"<str>"  + Userprefrenece.txt1 + "</str>";
            }
            elmXML.InnerXml = strConfiguration;
            xmlDoc.DocumentElement.AppendChild(elmXML);
            xmlDoc.Save(sFilename);
            MessageBox.Show("Saved Successfully");

<?xml version="1.0" encoding="utf-8" ?>
- <Testing>
- <Configuration>
  <str>NARAAYANAN</str>
  <str>DELL</str>
  <str>SUCCESS</str>
  </Configuration>
  </Testing>