Operation is not valid due to the current state of the object

Question

I have wrote code for getting dropdown based value from Sharepoint list.

But it give me error Operation is not valid due to the current state of the object when i set it to textbox.

Please take look of my code.

public void group2_Validating(object sender, XmlValidatingEventArgs e) { // Write your code here. XPathNavigator elementddlCostCenter = this.MainDataSource.CreateNavigator().SelectSingleNode("//my:myFields/my:group1/my:group2/my:ddlFirstName", this.NamespaceManager);

        string ddlDepartedFrom = elementddlCostCenter.Value;

        XPathNavigator elementddlArrivalAt = this.MainDataSource.CreateNavigator().SelectSingleNode("//my:myFields/my:group1/my:group2/my:ddlLastName", this.NamespaceManager);

        string ddlArrivalAt = elementddlArrivalAt.Value;

        if (ddlDepartedFrom != null && ddlDepartedFrom != "" && ddlArrivalAt != null && ddlArrivalAt != "")
        {
            // ListWebServices.Lists listServices = new ListWebServices.Lists();



            SPQuery qry = new SPQuery();

            string camlquery = "<Where><And><Eq><FieldRef Name='Title' /><Value Type='Text'>" + ddlDepartedFrom + "</Value></Eq>" + "<Eq><FieldRef Name='Categories' /><Value Type='Text'>" + ddlArrivalAt + "</Value></Eq></And></Where>";
            // string camlquery = "<Where><Eq><FieldRef Name='Categories'/><Value Type='Text'>"+ ddlArrivalAt +"</Value></Eq></Where>";
            qry.Query = camlquery;
            SPSite site = null;
            SPWeb web = null;
            try
            {

                SPSecurity.RunWithElevatedPrivileges(
                delegate()
                {
                    string m_sharepointUrl = "http://ProductionServer";
                    using (site = new SPSite(m_sharepointUrl))
                    {
                        if (site != null)
                        {

                            using (web = site.OpenWeb())
                            {
                                SPList list = web.Lists["Age"];

                                if (list != null)
                                {
                                    DataTable _table = new DataTable();
                                    DataColumn dc = new DataColumn("Mileage");
                                    _table.Columns.Add(dc);
                                    SPListItemCollection listItemsCollection = list.GetItems(qry);
                                    foreach (SPListItem item in listItemsCollection)
                                    {
                                        DataRow _row = _table.NewRow();
                                        _row[0] = SPEncode.HtmlEncode(item["Age"].ToString());
                                        Milage = Convert.ToInt32(SPEncode.HtmlEncode(item["Age"].ToString()));
                                        //Mileage = Convert.ToInt32(_row[0].ToString());
                                        _table.Rows.Add(_row);

                                    }

                                    XPathNavigator domNav = MainDataSource.CreateNavigator();
                                    string xhtmlRTFField = domNav.SelectSingleNode("//my:myFields/my:group1/my:group2/my:txtAge", NamespaceManager).InnerXml;

                                     domNav.SelectSingleNode("//my:myFields/my:group1/my:group2/my:txtAge", NamespaceManager).InnerXml = Milage.ToString(); ---> I got error here
                                    //domNav.SelectSingleNode("//my:myFields/my:group1/my:group2/my:txtAge", NamespaceManager).SetValue(Milage.ToString());
                                }
                                web.Close();
                            }
                            site.Close();
                        }
                    }

                }
                    );
            }
            catch (Exception ex)
            {
            }

        }
    }

Please help on this.

Thank you.