title: Dropdownlist in UpdatePanel in UserControl is not working
body: <p>I have this code inside a UserControl:</p>
  
  <p>ControlloUtente.ascx</p>
  
  <pre><code>&lt;td&gt;
      &lt;asp:UpdatePanel ID="up" runat="server"&gt;
          &lt;Triggers&gt;
              &lt;asp:AsyncPostBackTrigger ControlID="ddlE" EventName="SelectedIndexChanged" /&gt;
          &lt;/Triggers&gt;
          &lt;ContentTemplate&gt;
              &lt;asp:DropDownList ID="ddlE" AutoPostBack="true" runat="server" OnSelectedIndexChanged="ddlE_SelectedIndexChanged"&gt;
                  &lt;asp:ListItem Text="Text1" Value="1" Selected="True"&gt;&lt;/asp:ListItem&gt;
                  &lt;asp:ListItem Text="Text2" Value="2"&gt;&lt;/asp:ListItem&gt;
                  &lt;asp:ListItem Text="Text3" Value="3"&gt;&lt;/asp:ListItem&gt;
              &lt;/asp:DropDownList&gt;
          &lt;/ContentTemplate&gt;
      &lt;/asp:UpdatePanel&gt;
  &lt;/td&gt;
  </code></pre>
  
  <p>The UserControl is inside antoher UserControl:</p>
  
  <p>ListaControlli.ascx</p>
  
  <pre><code>&lt;%@ Register Src="~/BlaBla.ascx" TagName="BlaBla"
  TagPrefix="UcBla" %&gt;
  &lt;%@ Register Src="~/ControlloUtente.ascx" TagName="ControlloUtente"
  TagPrefix="UcCon" %&gt;
  &lt;%@ Register Src="~/BlaBla2.ascx" TagName="BlaBla2" 
  TagPrefix="UcBla2" %&gt;
  </code></pre>
  
  <p>That is in a page that already have an UpdatePanel:</p>
  
  <p>Pagina.aspx</p>
  
  <pre><code>    &lt;%@ Register Src="~/AltroUserControl.ascx" TagName="AUC" TagPrefix="ucAuc" %&gt;
      &lt;%@ Register Src="~/ListaControlli.ascx" TagName="Controlli" TagPrefix="uclControlli" %&gt;
  &lt;asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server"&gt;
  &lt;/asp:Content&gt;
  &lt;asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent"&gt;
  
      &lt;asp:UpdatePanel ID="upPagina" runat="server" ChildrenAsTriggers="true"&gt;
          &lt;ContentTemplate&gt;
              (...)
              &lt;uclControlli:Controlli ID="ucControlli" runat="server" /&gt;
              (...)
          &lt;/ContentTemplate&gt;
          &lt;Triggers&gt;
              &lt;asp:PostBackTrigger ControlID="ucAuc" /&gt;
          &lt;/Triggers&gt;
      &lt;/asp:UpdatePanel&gt;
  &lt;/asp:Content&gt;
  </code></pre>
  
  <p>But changing value of the dropdownlist I'm always having a full postback. There's something wrong?</p>