xml file create dynamically and insert record and display in gridview

Aspx.cs

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Xml;
using System.IO;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            fill();
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (!File.Exists(Server.MapPath("Studentdetail.xml")))
        {
            writefile();
        }
        else
        {
            readfile();
        }
    }

    public void writefile()
    {
        string path = Server.MapPath("Studentdetail.xml");

        XmlTextWriter ob = new XmlTextWriter(path, null);

        ob.WriteStartDocument();

        ob.WriteStartElement("Dataset");
        ob.WriteStartElement("Items");
        ob.WriteElementString("ID", txtid.Text);
        ob.WriteElementString("UserName", txtuname.Text);
        ob.WriteElementString("Password", txtpwd.Text);
        ob.WriteEndElement();
        ob.WriteEndElement();

        ob.WriteEndDocument();
        fill();
    }

    public void fill()
    {
        DataSet ds = new DataSet();

        ds.ReadXml(Server.MapPath("Studentdetail.xml"));

        GridView1.DataSource = ds;
        GridView1.DataBind();
    }

    public void readfile()
    {
        XmlDocument xdoc = new XmlDocument();
        xdoc.Load(Server.MapPath("Studentdetail.xml"));

        XmlElement Item = xdoc.CreateElement("Items");
        XmlElement i = xdoc.CreateElement("i");

        XmlElement id = xdoc.CreateElement("ID");
        XmlText xid = xdoc.CreateTextNode(txtid.Text);

        XmlElement uname = xdoc.CreateElement("UserName");
        XmlText xuname = xdoc.CreateTextNode(txtuname.Text);

        XmlElement pwd = xdoc.CreateElement("Password");
        XmlText xpwd = xdoc.CreateTextNode(txtpwd.Text);

        id.AppendChild(xid);
        uname.AppendChild(xuname);
        pwd.AppendChild(xpwd);


        Item.AppendChild(id);
        Item.AppendChild(uname);
        Item.AppendChild(pwd);

        xdoc.DocumentElement.AppendChild(Item);
        xdoc.Save(Server.MapPath("Studentdetail.xml"));
        fill();
    }
}

Studentdetail.xml

<?xml version="1.0" encoding="utf-8"?>
<Dataset>
  <Items>
    <i>
      <ID>asdf</ID>
      <UserName>asdf</UserName>
      <Password>df</Password>
    </i>
  </Items>
  <Items>
    <i>
      <ID>3</ID>
      <UserName>123</UserName>
      <Password>123</Password>
    </i>
  </Items>
  <Items>
    <ID>1</ID>
    <UserName>2</UserName>
    <Password>3</Password>
  </Items>
  <Items>
    <ID>hitesh</ID>
    <UserName>p</UserName>
    <Password>p</Password>
  </Items>
  <Items>
    <ID>1</ID>
    <UserName>hitesh</UserName>
    <Password>pass</Password>
  </Items>
  <Items>
  </Items>
  <Items>
  </Items>
</Dataset>