Navigation loop and clearing back-stack - windows

Question

Right now, what I have is, when the user clicks on the page, the page will automatically send the user to a webtask which opens up a pdf file.
What's happening right now is when the user presses the back button, it goes back to the Original page for a split second, before being redirected back to the pdf as I have assigned it to (due to the onnavigateto function)
How would I make it so that, when the user clicks the back button in the pdf document, the app will take the user back to the main page?
Also, on the main page, how do I ensure that the backstack is cleared? (As the Application HAS to exit on the MainPage, so can't go back to the pdf.)
My Code so far, I have tried...
{
public partial class Page2 : PhoneApplicationPage
{
public Page2()
{
InitializeComponent();
}
//as soon as this page is opened, navigate/redirect it to the URL below
protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
base.OnNavigatedTo(e);
WebBrowserTask task = new WebBrowserTask() { URL ="http://test.com/test.pdf"};
task.Show();
}
//when the user clicks the hardware back button, instead of taking them to the daily notices, which will send them back to brower
// send the user to the main page
protected override void OnBackKeyPress
(System.ComponentModel.CancelEventArgs e)
{
base.OnBackKeyPress(e);
new Uri("/MainPage.xaml", UriKind.Relative);
}

Answer 1

First of all, why you need the second page that only opens a WebBrowserTask? You can do this from main page.
If you still want to open from second page, you can move WebBrowserTask to constructor and surround it with Dispatcher. This approach is guaranteed that WebBrowserTask will be called only once after navigation to this page (maybe will be some problems with tombstoning). Or, you can save state to PhoneApplicationPage.State to handle where user was and what you should open next.
For clearing back stack you can use next code:
while (NavigationService.BackStack.Any())
{
NavigationService.RemoveBackEntry();
}

Answer 2

You will have to detect this on the application level, rather than the page level. When you 'redirect' the user to the PDF, your application becomes suspended. When they then navigate back, it is resumed.
The Visual Studio template provides a method that it invoked when the application resumes:
// Code to execute when the application is activated (brought to foreground)
// This code will not execute when the application is first launched
private void Application_Activated(object sender, ActivatedEventArgs e)
{
}
Within the above method, you could set a flag, that you then check when your page is navigated to,, that indicates a resume has occurred.