InfoPath Dev

This is my first InfoPath form.  We have InfoPath as part of our desktop install.  SharePoint is coming but will not be here until next year.  I also have VSTA installed.  I state this in case it makes a difference on this post.

I have the following fields/containers within the myfields container:

AcademicPerformance (Check box)   
AcademicPerformanceGroup
    CurrentGrade*
    Tutor (Check box)
    TutorGroup
        TutorName
        TutorPersonID

On the AcademicPerformanceGroup I have conditional formatting which hides the section if the AcademicPerformance check box is false.  What I have noticed is that if a "form filler" enters info into the Academic Performance and Tutor groups and then uncheck the Academic Performance check box the section is collapsed but the data remains.  I would like the default behavior of unchecking the box to throw up a warning (next programming iteration) and then clear the subordinate fields before collapsing.

This is what I have come up with so far:

using Microsoft.Office.InfoPath;
using System;
using System.Windows.Forms;
using System.XML;
using System.Xml.XPath;
using mshtml;

namespace StudentReferral
{
  public partial class FormCode
  {
    private string initialAPData;
    public void InternalStartup()
    {
      EventManager.XmlEvents["/my:myFields/my:AcademicPerformance"].Changing += new XmlChangingEventHandler(AcademicPerformance_Changing);
      EventManager.FormEvents.Loading += new LoadingEventHandler(FormEvents_Loading);
    }
    public void AcademicPerformance_Changing(object sender, XmlChangingEventArgs e)
    {
      String fieldValue = MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:AcademicPerformance", NamespaceManager).Value;
      if (fieldValue == "false")
      {
MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:AcademicPerformanceGroup", NamespaceManager).ReplaceSelf(initialAPData);
      }
    }

    public void FormEvents_Loading(object sender, LoadingEventArgs e)
    {
      initialAPData = MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:AcademicPerformanceGroup", NamespaceManager).OuterXml;
    }
  }
}

First of all, is there a better way of clearing several containers/fields within a section(s) rather than resorting to code? 
If not, I am getting a little error that if someone can see I would greatly appreciate it.  Once I get beyond the error I plan to add an ok/cancel dialog and then add the code to all my expanding sections.

When the "MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:AcademicPerformanceGroup", NamespaceManager).ReplaceSelf(initialAPData);" statement executes I receive the following error:

A first chance exception of type 'System.InvalidOperationException' occurred in Microsoft.Office.InfoPath.Client.Internal.Host.Interop.dll

InitialAPData (after the form load event) =
<my:AcademicPerformanceGroup xmlns:my="http://schemas.microsoft.com/office/infopath/2003/myXSD/2009-09-28T17:12:30">
  <my:CurrentGrade></my:CurrentGrade>
  <my:Tutor>false</my:Tutor>
  <my:TutorGroup>
   <my:TutorName></my:TutorName>
   <my:TutorPersonID xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"></my:TutorPersonID>
  </my:TutorGroup>
 </my:AcademicPerformanceGroup>
 
Any ideas why this error would be encountered? 

 InfoPath will not allow you to change the contents of the data source in a Changing event.  Please try adding that code to AcademicPerformance's Changed event.

Thanks for your reply Jimmy.

 I added the logic to the Changed event as you indicated.  It worked well. 

On to adding the other groups.  This was not too successful.  I have to spend some time to see what I did wrong; not sure what, but all I did was copy the code and paste the names in from xcopy. 

 I also looked at the ok/cancel functionality.  So far I can't find out how to instantiate xdocument to get at the UI.  It looks like it should be automatic but under VS2008/InfoPath 2007 it is not.  I'm not familiar enough with the API to know if it was moved, changed, or what.

 Anyway, a big thank you for making my day brighter with the tibit you passed along.

Steve

The other groups are now working too with a little help with the debugger. 

I'm still stuck on how to throw up a ok/cancel that the fields are about to be cleared by collapsing a section.  If you can shed any light on how to do this it would be appreciated.  Note environment on my last post...

 Are you saying you're looking for the thisXDocument.UI.confirm() function?

In IP 2007 you should just use the usual System.Windows.Forms API:

DialogResult response = MessageBox.Show("Are you sure you want to do this?", string.Empty,
     MessageBoxButtons.YesNo);
if (response == DialogResult.Yes)
{
     // Do stuff
}

Thanks again Jimmy; you've been very helpful!