title: Trigger an update of the UpdatePanel by a control that is in different ContentPlaceHolder
body: <p>I have a page with two ContentPlaceHolders.  One has a DropDown and another UpdatePanel with content. </p>
  
  <p>How can I trigger update to the UpdatePanel by the DropDown's selectedItemChanged event when they are in different ContentPlaceholders?</p>
  
  <p>The following would not work since UpdatePanel1 doesn't know about DropDown1:</p>
  
  <pre><code>&lt;asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"  ChildrenAsTriggers="true"&gt;
      &lt;ContentTemplate&gt;
          Some content that needs to be updated here...
      &lt;/ContentTemplate&gt;
      &lt;Triggers&gt;
          &lt;asp:AsyncPostBackTrigger ControlID="DropDown1" EventName="SelectedIndexChanged" /&gt;
      &lt;/Triggers&gt;
  &lt;/asp:UpdatePanel&gt;
  </code></pre>
  
  <p>One way is to make an ajax page method that would be called by javascript on the page when DropDown's item is selected. Then in code behind, inside that page method, call UpdatePanel1.Update(). </p>
  
  <p>Is there an easier alternative?</p>