Edit: Okay, now, having carefully read your question, I can give you a better answer.
Firstly, you cannot pass data to the page constructor, since the runtime itself handles instantiation during navigation, and you can only navigate using the NavigationService . However, you have other options.
One of them uses a query string, but if you do not want to use this, you can use PhoneApplicationService to store a complex object and read another page.
In addition, global variables are used. These are the only options I can think of.
To go to a page in your ProjectB class library, you need to use a uri similar to the following:
/{assemblyName};component/{path}
In your case, you can do the following:
NavigationService.Navigate(new Uri("/ProjectB;component/MyPageInProjectB.xaml", UriKind.Relative));
Please note that I am specifying the relative path here, so MyPageInProjectB.xaml must be in the root folder inside ProjectB for the above example to work.
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