title: PreviousPage = null in crosspage posting
body: <p>I'm trying to pass post data from one page to the next but PreviousPage always equals null.</p>
  
  <p>This is what I have in the original page (inside an updatepanel
  )</p>
  
  <pre><code>&lt;asp:Button ID="mmGo" runat="server" Text="Merge" PostBackUrl="~/Default2.aspx?action=merge"/&gt;
  </code></pre>
  
  <p>Then in the following page I have:</p>
  
  <pre><code>&lt;%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %&gt;
  &lt;%@ PreviousPageType VirtualPath="~/Default.aspx" %&gt;
  
  &lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
  
  &lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
  &lt;head runat="server"&gt;
      &lt;title&gt;&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;form id="form1" runat="server"&gt;
  
      &lt;div&gt;
          &lt;asp:Label ID="Label1" runat="server" Text="Label"&gt;&lt;/asp:Label&gt;
      &lt;/div&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>...and in the .cs file:</p>
  
  <pre><code>protected void Page_Load(object sender, EventArgs e)
  {
      if (PreviousPage != null)
      {
          string action = Request.QueryString["action"];
          if (action == "merge")
          {
              UpdatePanel SourceControl = (UpdatePanel)PreviousPage.FindControl("UpdatePanel1");
              HiddenField SourceTextBox = (HiddenField)SourceControl.FindControl("txtListIDs");
              if (SourceTextBox != null)
              {
                  Label1.Text = SourceTextBox.Value;
              }
          }
      }
      else
      {
          Label1.Text = "page is not cross page post back!";
      }
  }
  </code></pre>
  
  <p>Any ideas why the PreviousPage is always equal to null?</p>