AutoCompleteExtender not working

<asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server"  MinimumPrefixLength="1"
        ServiceMethod="GetInfo" ServicePath="~/WebService.asmx"
        TargetControlID="Txt_searchbox"></asp:AutoCompleteExtender>

public string[] GetInfo( string  a)
   {
       bl.id = a;
       DataTable dt = bl.Name();
        string [] user= new string [ dt.Rows .Count ];
        for (int i = 0; i < dt.Rows.Count; ++i)
        {
            user[i] = Convert.ToString(dt.Rows[i]["City_Name"]);
        }
        return user.ToArray();

   }