title: UpdatePanel Trigger Error. Could not find an event on associated control for the trigger in UpdatePanel
body: <p>I am having trouble with asp:UpdatePanel and Trigger of ASP.NET WebForms. Trigger could not find the event on my UpdatePanel. I've seen a lot of examples and I copied their implementations but could not get it right. I'm totally new in WebForms. Please help. Thank you.</p>
  
  <pre><code>                &lt;tr&gt;
                      &lt;td class="label1"&gt;Will use ETL?&lt;/td&gt;
                      &lt;td&gt;
                          &lt;asp:UpdatePanel runat="server" UpdateMode="Conditional" ID="updatePanel1"&gt;
                              &lt;ContentTemplate&gt;
                                  &lt;asp:RadioButtonList ID="useEtl" runat="server" RepeatDirection="Horizontal" Width="120" OnSelectedIndexChanged="useEtl_SelectedIndexChanged"&gt;
                                      &lt;asp:ListItem Text="Yes" Value="1" /&gt;
                                      &lt;asp:ListItem Text="No" Value="0" /&gt;
                                  &lt;/asp:RadioButtonList&gt;
                              &lt;/ContentTemplate&gt;
                          &lt;/asp:UpdatePanel&gt;
                      &lt;/td&gt;
                      &lt;td class="label1"&gt;ETL Box&lt;/td&gt;
                      &lt;td&gt;
                          &lt;asp:UpdatePanel runat="server" UpdateMode="Conditional" ID="updatePanel2"&gt;
                              &lt;ContentTemplate&gt;
                                  &lt;asp:TextBox ID="etlBox" runat="server"/&gt;
                              &lt;/ContentTemplate&gt;
                              &lt;Triggers&gt;
                                   &lt;asp:AsyncPostBackTrigger ControlID="useEtl" EventName="OnSelectedIndexChanged" /&gt;
                               &lt;/Triggers&gt;
                          &lt;/asp:UpdatePanel&gt;
                      &lt;/td&gt;
                  &lt;/tr&gt;
  </code></pre>