protected void AddEmailButton_Click(object sender, EventArgs e)
    {

        string filename = "LetterTest.xml";

        //open file to update
        FileStream fs = new FileStream(filename, FileMode.Open);
        
        //writer setting
        XmlWriterSettings xmlW = new XmlWriterSettings();
        xmlW.CheckCharacters = true;
        xmlW.Indent = true;

        XmlDocument document = new XmlDocument();
        document.Load(fs);       
        XPathNavigator navigator = document.CreateNavigator();
        navigator.MoveToRoot(); //root
        navigator.MoveToFollowing("Newsletter", ""); //Newsletter element
        var wnav = navigator.AppendChild();
        //create instance of writer
        XmlWriter writer = XmlWriter.Create(wnav, xmlW);

        //IvalidOperation
        //WriteStartDocument cannot be called on writers created with ConformanceLevel.Fragment.
        writer.WriteStartElement("Member");

        writer.WriteStartElement("email");
        writer.WriteValue(AddEmailTextBox.Text.ToString());
        writer.WriteEndElement(); //email

        writer.WriteStartElement("create_date");
        writer.WriteValue(DateTime.Now.ToShortDateString());
        writer.WriteEndElement(); //create_date
   
        writer.WriteEndElement(); //Member

        writer.Close();
        //finished writing, file updated and closed

    }