/// <summary>
/// Handles structure-changed events. If a new app window has been added, this method ensures
/// it's in the list of runtime IDs and subscribed to window-close events.
/// </summary>
/// <param name="sender">Object that raised the event.</param>
/// <param name="e">Event arguments.</param>
/// <remarks>
/// An exception can be thrown by the UI Automation core if the element disappears
/// before it can be processed -- for example, if a menu item is only briefly visible. 
/// This exception cannot be caught here because it crosses native/managed boundaries. 
/// In the debugger, you can ignore it and continue execution. The exception does not cause
/// a break when the executable is being run.
/// </remarks>
private void OnStructureChanged(object sender, StructureChangedEventArgs e)
{
    AutomationElement element = sender as AutomationElement;

    if (e.StructureChangeType == StructureChangeType.ChildAdded)
    {
        Object windowPattern;
        if (false == element.TryGetCurrentPattern(WindowPattern.Pattern, out windowPattern))
        {
            return;
        }
        int[] rid = e.GetRuntimeId();
        if (RuntimeIdListed(rid, savedRuntimeIds) < 0)
        {
            AddToWindowHandler(element);
            savedRuntimeIds.Add(rid);
        }
    }
}

// elementRoot is an AutomationElement.
Automation.AddStructureChangedEventHandler(elementRoot, TreeScope.Children, 
    new StructureChangedEventHandler(OnStructureChanged));