Is AsyncPostBackTrigger not a known item?

Question

I am using asp.net and C #, creating a page with ajax service packs. When I try to insert a trigger element, I get an error message AsyncPostBackTrigger is not a known element. What am I missing?

<asp:UpdatePanel ID="UdpEPL" runat="server" UpdateMode="Conditional" Visible="False"> <ContentTemplate> <Triggers> <asp:AsyncPostBackTrigger ControlID="BtnEplShowSubmit" EventName="BtnEplShowSubmit_Click"/> </Triggers> </ContentTemplate> </asp:UpdatePanel> 

Answer 1

Remove the Triggers section from the ContentTemplate :

 <asp:UpdatePanel ...> <Triggers> <asp:AsyncPostBackTrigger .../> </Triggers> <ContentTemplate> </ContentTemplate> </asp:UpdatePanel>