NavigationService.Navigate抛出NullReferenceException WP7？

Question

Just Downloaded the WP7 SDK yestarday - And I'm new to Silver light - and Application development in general.

I'm just simply trying to Direct my user to a different page, but I have a NullRefernceException Error occur on a Line Of Code. The Stupid thing - the code above it is exactly the same - yet doesn't throw the error?

if (myISO.DirectoryExists("Logs") && myISO.DirectoryExists("DataStore"))
{
     NavigationService.Navigate(new Uri("Sign-in.xaml", UriKind.Relative));
}
else
{
     NavigationService.Navigate(new Uri("Welcome.xaml", UriKind.Relative));
     //Above Line Causes Exception^
}

Also I get an NullRefernceException in App.xaml.cs - which I haven't even changed - it WAS working - now it doesn't :

PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Disabled;

Answer 1

it should be like this

NavigationService.Navigate(new Uri("/Page1.xaml", UriKind.Relative));

where is your "/" ?

Answer 2

Can you debug and identify which part of line throws an exception?

Also if you use NavigationService.Navigate(new Uri("/Welcome.xaml", UriKind.Relative)); with "/" make sure that Welcome.xaml page is located in a root directory of the project.

Answer 3

Actually,

This problem occurs after you change the Assembly Name or Namespace in 
Project Properties.

To fix it, Delete all contents of the Bin and obj Folders and recompile. see this thread .

Also this is probably the reason why it worked when you created a new project.