In Aspx

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true">

        <Services>

            <asp:ServiceReference Path="~/WebServices/AutoComplete.asmx" />

        </Services>

    </asp:ScriptManager>

  <asp:TextBox ID="txtName" TabIndex="3" runat="server" CssClass="TextBox"

                        Width="250px" MaxLength="100"></asp:TextBox>

                    <cc1:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" Enabled="true"

                        EnableCaching="true" MinimumPrefixLength="1" TargetControlID="txtName"

                        ServiceMethod="GetNames" ServicePath="~/WebServices/AutoComplete.asmx">

                    </cc1:AutoCompleteExtender>

In asmx.cs file

using System;

using System.Web;

using System.Collections;

using System.Web.Services;

using System.Web.Services.Protocols;

using System.Collections.Generic;

using System.Web.Script.Services;

using System.Data;

[ScriptService()]

[WebService(Namespace = "http://tempuri.org/")]

[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]

public class AutoComplete : System.Web.Services.WebService

{

    [WebMethod]

    [ScriptMethod()]

    public string[] GetNames(string prefixText, int count)

    {

        ArrayList filteredList = new ArrayList();

        MySQLConnection con = new MySQLConnection();

        string qry = "SELECT column_name FROM table_name WHERE column_name LIKE '%" + prefixText + "%' ";

        DataSet ds;

        ds = con.getDataset(qry);

        foreach (DataRow dr in ds.Tables[0].Rows)

        {            if(dr["column_name"].ToString().ToLower().StartsWith(prefixText.ToLower()))

                filteredList.Add(dr["column_name "].ToString());

        }

        con.closeConnection();

       

        return (string[])filteredList.ToArray(typeof(string));

    }

}