AutoComplete in ASP.net C# Ajax

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeFile="Default.aspx.cs" Inherits="_Default" %>
     <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <asp:ScriptManager ID="ScriptManager1" runat="server">

    </asp:ScriptManager>

 

    <asp:TextBox ID="txtCity" runat="server"></asp:TextBox>

    <asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" TargetControlID="txtCity"

         MinimumPrefixLength="1" EnableCaching="true" CompletionSetCount="1" CompletionInterval="1000"

         ServiceMethod="GetCity" >

    </asp:AutoCompleteExtender>

</asp:Content>

///

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Data;

using System.Configuration;

using System.Data.SqlClient;

public partial class _Default : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

    }

    [System.Web.Script.Services.ScriptMethod()]

    [System.Web.Services.WebMethod]

    public static List<string> GetCity(string prefixText)

    {

        DataTable dt = new DataTable();

        string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ToString();

        SqlConnection con = new SqlConnection(constr);

        con.Open();

        SqlCommand cmd = new SqlCommand("select * from TblCountry where CountryName like @City+'%'", con);

        cmd.Parameters.AddWithValue("@City", prefixText);

        SqlDataAdapter adp = new SqlDataAdapter(cmd);

        adp.Fill(dt);

        List<string> CityNames = new List<string>();

        for (int i = 0; i < dt.Rows.Count; i++)

        {

            CityNames.Add(dt.Rows[i][1].ToString());

        }

        return CityNames;

    }

}