Collection View not showing in master detail page

why is it that when l use the following code in my master detail page app.xaml.cs the collection view on the page does not show

 public App()
        {
            MainPage = new comicAppFinal.MainPage();
        }

but when the code below the collection view shows but l can not access the masterdetail page functionality anymore.

public App()
{
   MainPage = new Navigation(new MainPage())
}