validar usuario y decidir que pagina abrir de app.xaml.cs

public static void validateUser(string username, int password) {
    if (username.Equals("juan") && password == 123) 
        Application.Current.MainPage = new Page1();
    else if (username.Equals("pedro") && password == 456)
        Application.Current.MainPage = new Page2();
    else if (username.Equals("maria") && password == 789)
        Application.Current.MainPage = new Page3();
    else
        Application.Current.MainPage = new MainPage();
}