ASP.Net 向xml文件中添加数据(insert操作)

try
            {
                XmlDocument doc = new XmlDocument();
                string filename = Server.MapPath(".//XML//User.xml");
                doc.Load(filename);
                XmlNode root = doc.DocumentElement;
                XmlNode user = doc.CreateElement("user");

                DataSet ds = new DataSet();
                ds.ReadXml(filename); 
                int userid = ds.Tables[0].Rows.Count+1;
                               //写入userid
                XmlElement elem6 = doc.CreateElement("userid");
                elem6.InnerText = userid.ToString();
                user.AppendChild(elem6);
                //写入用户名
                XmlElement elem1 = doc.CreateElement("username");
                elem1.InnerText = this.Txt_username.Text.Trim();
                user.AppendChild(elem1);
                //写入密码
                XmlElement elem2 = doc.CreateElement("userpass");
                elem2.InnerText = this.Txt_pasw.Text.Trim();
                user.AppendChild(elem2);
                //写入Email
                XmlElement elem3 = doc.CreateElement("usermail");
                elem3.InnerText = this.Txt_usermail.Text.Trim();
                user.AppendChild(elem3);
                //写入QQ
                XmlElement elem4 = doc.CreateElement("usersite");
                elem4.InnerText = this.Txt_siteurl.Text.Trim();
                user.AppendChild(elem4);
                //写入MSN
                XmlElement elem5 = doc.CreateElement("usermsn");
                elem5.InnerText = this.Txt_msn.Text.Trim();
                user.AppendChild(elem5);
                //写入日期
                XmlElement elem8 = doc.CreateElement("ndate");
                elem8.InnerText = DateTime.Now.ToString();
                user.AppendChild(elem8);
                
                root.AppendChild(user);
                doc.Save(filename);    
                this.Lab_flag.Text = "注册成功!";
                this.HyperLink1.Visible=true;
                this.Txt_username.Text="";
                this.Txt_pasw.Text="";
                this.Txt_firpasw.Text="";
                this.Txt_usermail.Text="";
                this.Txt_msn.Text="";
                this.Txt_siteurl.Text="";
                return true;
               
            }
            catch(Exception ex)
            {
                this.Lab_flag.Text = ex.Message;
                return false;
            }