title: when I use the DropDownList1_SelectedIndexChanged to be the trigger of UpDatePanel，it dosn't work
body: <p>And the tip is the “DropDownList1” didn't find “DropDownList1_SelectedIndexChanged”
  </p>
  
  <pre><code>&lt;/asp:ScriptManager&gt;
  
  &lt;asp:UpdatePanel ID="UpdatePanel1" runat="server"&gt;
  
     &lt;ContentTemplate&gt;
  
      &lt;asp:DropDownList ID="DropDownList1" runat="server" 
  
          onselectedindexchanged="DropDownList1_SelectedIndexChanged"&gt;
  
          &lt;asp:ListItem&gt;1&lt;/asp:ListItem&gt;
  
      &lt;/asp:DropDownList&gt;
  
     &lt;/ContentTemplate&gt;
  
          &lt;Triggers&gt;
  
              &lt;asp:AsyncPostBackTrigger ControlID="DropDownList1" EventName="DropDownList1_SelectedIndexChanged" /&gt; 
  
          &lt;/Triggers&gt;
  </code></pre>
  
  <p>
  Please tell me How to make it work!</p>