System.Web.UI.Controls.ControlCollection.Add Throws ArgumentOutOfRangeException

I'm trying to make a tabbed user interface using an asp:Panel and the asp:TreeView. I'm keeping both TreeViews initialized in the background and when I need to display one, I clear the panel and then add the appropriate TreeView.

I'm keeping the TreeView's stored in the Session variable so they don't go away when the page gets reloaded due to a postback.

But when I try to add the control, I get an ArgumentOutOfRangeException. Here's the small snippet of code that matters:

<asp:UpdatePanel UpdateMode="Conditional" runat="server">
  <ContentTemplate>
    <asp:LinkButton ID="ContentButton" OnClick="ContentButton_Click" Text="Contents" runat="server" />
  </ContentTemplate>
</asp:UpdatePanel>

<asp:UpdatePanel UpdateMode="Conditional" runat="server">
  <Triggers>
    <asp:AsyncPostBackTrigger ControlID="ContentButton" />
  </Triggers>
  <ContentTemplate>
    <div id="TOCPanel" runat="server" />
  </ContentTemplate>
</asp:UpdatePanel>

Here's the related code in the C# file.

protected void IndexButton_Click( object sender, EventArgs e )
{
    TOCPanel.Controls.Clear();
    // _TreeView points to a property that retrieves the TreeView from Session["TreeView"]
 开发者_Go百科   TOCPanel.Controls.Add( _TreeView ); // This line fails
    // TOCPanel.Controls.Add( new TreeView() ); // This fails with the same error message too
}

Is there any reason

Not sure what the problem is, but I'd suggest adding both TreeViews to the ContentTemplate, and then hiding/showing them with the Visible property. This gets rid of your dependency on Session as well. I've found manually adding and removing controls gets messy, especially when dealing with ViewState.