autocomplete textbox in asp.net webservice

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ASP.NET

I use master page and use script to open a popup window.

This popupwindow have one textbox.
I add a ajax autocomplete extender in textbox.
It run and get the values and I check the breakpoint.
But not display the autovalues in textbox.
How to do solve this problem?

Advance thanks.....

<asp:TextBox ID="txtshare" Width="125px" MaxLength="50" runat="server">
                          
<ajaxToolkit:autocompleteextender  
        ID="aceSearch"  
        runat="server" 
        TargetControlID="txtshare" 
        MinimumPrefixLength="1" 
        EnableCaching="true" 
        CompletionSetCount="5" 
        CompletionInterval="100" 
        ServiceMethod="GetUsers" 
        ServicePath="CloudlistService.asmx" 
        CompletionListCssClass="field small-field" >

[WebMethod]
    public string[] GetUsers(string prefixText, int count)
    {
       
        //ADO.Net
        //string a=Session["ln"].ToString();
        //SqlConnection cn = new SqlConnection();
        con.Close();
        DataSet ds = new DataSet();
        DataTable dt = new DataTable();
        //String strCn = "Data Source=.\\sqlexpress;Initial Catalog=cloudlist;Integrated Security=True";
        ///cn.ConnectionString = strCn;
        SqlCommand cmd = new SqlCommand();

        //cmd.CommandType = CommandType.Text;


        string cmdtext = "select Username from users Where  Username like @myParameter+'%'";

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

        //cmdtext += " and Listname = @Listname";
        //cmd.Parameters.AddWithValue("@Listname", contextKey);

        cmd.CommandText = cmdtext;
        cmd.Connection = con;

        //cmd.Parameters.AddWithValue("@Listname", contextKey);
        try
        {
            con.Open();
            cmd.ExecuteNonQuery();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(ds);
        }
        catch
        {
        }
        finally
        {
            con.Close();
        }
        dt = ds.Tables[0];

        //Then return List of string(txtItems) as result
        List<string> userNames = new List<string>();

        String dbValues;

        foreach (DataRow row in dt.Rows)
        {
            //String From DataBase(dbValues)
            dbValues = row["Username"].ToString();

            dbValues = dbValues.ToLower();
            userNames.Add(dbValues);
        }

        return userNames.ToArray();
    }