Selected index changed of grid view not working inside update panel

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C#

ADO.NET

Hi,
I placed my grid view inside update panel and took it into content template. Than I picked up triggers of update panel and placed async post back trigger id to the id of grid view and named the event as selected event changed. Still I am not able to use it.
I have seen many threads in which people are making use of it than where am I going wrong ?

<asp:UpdatePanel ID="UpdatePanel2" runat="server">
                            <Triggers>
                            <asp:AsyncPostBackTrigger ControlID = "gv_workOrders" ControlID="SelectedIndexChanged" />
                            </Triggers>
                            <ContentTemplate>

<my grid view>
</ContentTemplate>
                                </asp:UpdatePanel>

On clicking on the link button inside the grid view I will redirect the user to another page, till he gets redirected I want show him loading image in update progress. That's why I am trying to do this.
But Its not getting done.
Please help.