title: Windows Phone 7 Sign in screen redirect
body: <p>Apologise if there is answer for this somewhere else, I can't find any.</p>
  
  <p>I imagine this is a common situation. The first time the user runs the application I want them to be presented with the SignIn.xaml, if they don't have any saved details, else I just want to go straight to the MainPage.xaml.</p>
  
  <p>I'd usually do this by sticking a check in the constructor of the MainPage and if they don't have details navigate away. The problem I have is that</p>
  
  <pre><code>NavigationService.Navigate(new Uri("/SignIn.xaml", UriKind.Relative));
  </code></pre>
  
  <p>is coming up as a null reference. What am I missing? Is there a proper way to do something like this in WP7?</p>
  
  <p>Thanks</p>