c#, file IO issue

I see alert message "Process cannot access the file because it is being used by another process".
when I click the button at third time. It seems reading is okay. but when i click the button to write something inside  to the xml.
it fails. how can i resolve this issue?

I know the  "Process cannot access the file because it is being used by another process". continue to happen the new message is sent.
sendMsg_Click(), but that does not mean the issues came from there.

And I check all controllers I used, and when the file is opened, it is closed immediately.

  protected void Page_Load(object sender, EventArgs e)
        {
            phoneNoList.DataSource = Utility.Controller.GetUniquePhoneNo(fromPhoneNo);
            phoneNoList.DataBind();
}

  protected void sendMsg_Click(object sender, EventArgs e)
        {
		 #region add a record to all message xml
                Domain.User u = new Domain.User();
                u.fullName = fullName.Text;
                u.from = "+1" + fromPhoneNo;
                u.to = "+1" + toPhoneNo.Text;
                u.body = body.Text;
                u.dateTimeSent = DateTime.Now.ToString();
                u.direction = "outbound-reply";
                string result = Utility.XML.Add(Utility.Controller.filePathUserMessagesAll, u);
                #endregion


}

  public static string Add(string inputFilePath,Domain.User u)
        {
            XDocument doc = XDocument.Load(inputFilePath);
            XElement user = new XElement("User",
                new XAttribute("PhoneNo",""),
                new XElement("FullName",u.fullName),
                new XElement("From",u.from),
                new XElement("To",u.to),
                new XElement("Body",u.body),
                new XElement("DateTimeSent",u.dateTimeSent),
                new XElement("Direction",u.direction),
                new XElement("Carrier",
                      new XElement("Name","Twilio"),
                      new XElement("SID",Utility.Controller.TwilioSID),
                      new XElement("Token",Utility.Controller.TwilioToken)));
            doc.Root.Add(user);
            doc.Save(inputFilePath);
            return "Added";
        }

    public static DataTable GetUniquePhoneNo(string accountPhoneNo)
        {
            string clientPhoneNo = string.Empty;
            string inputFilePath = filePathUserMessagesAll;
                //@"C:\Users\Documents\visual studio 2015\Projects\AppText\AppText\Data\UserMessagesAll.xml";
            DataTable dt = new DataTable();
            dt.Clear();
            dt.Columns.AddRange(new DataColumn[]
           {
                new DataColumn("PhoneNo",typeof(string)),
                new DataColumn("Client",typeof(string)),
                new DataColumn("DateCreated",typeof(string))
           });
            XDocument xdoc = null;
            using (XmlReader xr = XmlReader.Create(inputFilePath))
            {
                xdoc = XDocument.Load(xr);
                var query1 = from t in xdoc.Descendants("User")
                             where t.Element("From").Value.ToLower() == "+1" + accountPhoneNo.ToLower()
                             select new
                             {
                                 To = t.Element("To").Value,
                                 FullName = t.Element("FullName").Value,
                                 DateTimeSent = t.Element("DateTimeSent").Value
                                 //DateTime.Parse(txtDatepicker_end.Text).ToString("YYYY-MM-dd");
                             };
                var query2 = from t in xdoc.Descendants("User")
                             where t.Element("To").Value.ToLower() == "+1" + accountPhoneNo.ToLower()
                             select new
                             {
                                 To = t.Element("From").Value,
                                 FullName = t.Element("FullName").Value,
                                 DateTimeSent = t.Element("DateTimeSent").Value
                             };
                var query3 = query1.Concat(query2).OrderByDescending(z => z.DateTimeSent).GroupBy( z => z.To).Select(z=> z.First());
                foreach (var x in query3)
                {
                    dt.Rows.Add(x.To, x.FullName, (DateTime.Parse(x.DateTimeSent)).ToString("MM/dd/yy", System.Globalization.CultureInfo.InvariantCulture));
                }

               // string timeF = "2017-06-15T14:49:19-07:00";
               // DateTime dt = DateTime.Parse(timeF);
               // string timeS = dt.ToUniversalTime().ToString("MM/dd/yy hh:mm", System.Globalization.CultureInfo.InvariantCulture);


                // var messages2a = MessageResource.Read().Where(x => x.From.ToString() == "+1" + fromPhoneNo).GroupBy(x => x.To).Select(y => y.First());
                // var messages3a = MessageResource.Read().Where(y => y.To == "+1" + fromPhoneNo).GroupBy(x => x.To).Select(y => y.First());
                // var allz = messages2a.Concat(messages3a).OrderByDescending(z => z.DateCreated);
                return dt;
            }
        }

Select all Open in new window