return value from parent page and open popup page asp.net

<script type="text/javascript"> 
        function myDialogDuck(conid,openthispage){
        var control_id = conid.split("ctl00_ContentPlaceHolder1_TabContainer1_tpl1_");
        var real_id = control_id[1];
         returnvalue = window.showModalDialog(openthispage,'newWin','menubar:no;status:no;location:no;toolbar:no;dependent=no;dialogWidth:820px;dialogHeight:350px;');
          if (returnvalue != null){
          switch (real_id)
            {
            case "btn_scpa":document.getElementById('<%=ddl_scpa.ClientID%>').value = returnvalue;break;
            case "btn_add":document.getElementById('<%=ddl_add.ClientID%>').value = returnvalue;break;
            
            default:break;
            }
            return true;
          }
        }
    </script>

                                                                        <asp:UpdatePanel ID="UpdatePanel5" runat="server" RenderMode="Inline" UpdateMode="Conditional">
                                                                        <ContentTemplate>
                                                                            <asp:DropDownList ID="ddl_scpa" runat="server" Width="40px">
                                                                            </asp:DropDownList>
                                                                        </ContentTemplate>
                                                                        <Triggers>
                                                                            <asp:AsyncPostBackTrigger ControlID="btn_scpa" EventName="Click" />
                                                                        </Triggers>
                                                                    </asp:UpdatePanel>

    Private Sub OcReady()
        btn_scpa.Attributes.Add("OnClick", "myDialogDuck(this.id,'class.aspx?class=2');")
        btn_add.Attributes.Add("OnClick", "myDialogDuck(this.id,'class.aspx?class=3');")
    End Sub