ListView selectionMode in windows phone 8.1

Question

I'm having a problem in the development of a windows phone 8.1 app. The app in question presents a page that displays a listView. via a button in the appbar.

I switch from single to multiple selection (with checkboxes), and pressing the back_button returns to the default selection (as the messages app in windows phone).

Everything works correctly when I open the app, but if I navigate to other pages and then return to the page described above, then nothing works as it should.

The first function sets multiple selections. However, if I press the back button ( I'm in App7.MainPage) then the test MyListview.selectionmode == ListViewSelectionMode.none returns true.

public void MultiSelButton_Click(object sender, RoutedEventArgs e)
{
    MyListview.SelectionMode = ListViewSelectionMode.Multiple;
    BottomAppBar = appbars.MultiSelBar;                               
}

void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
{
    Frame rootFrame = Window.Current.Content as Frame;
    String sourcepage = null;

    if (rootFrame != null)
    {
        sourcepage = rootFrame.CurrentSourcePageType.ToString();

        switch (sourcepage)
        {                    
            case "App7.MainPage":
                if (MyListview.SelectionMode == ListViewSelectionMode.None)
                {
                     rootFrame.BackStack.Clear();
                     return;
                 }
                 torna_single();
                 break;

             case "App7.Cerca":
                   Frame.Navigate(typeof(MainPage));
                   break;

              case "App7.InserisciLeggiNota":
                   if (Frame.CanGoBack)
                       Frame.GoBack();
                   break;
          }

         e.Handled = true;
     }
}