MainActivity.OnBackPressed.cs

// Confirm with dialog
public override void OnBackPressed()
{
    if (((ConfirmBack.App)App.Current).PromptToConfirmExit)
    {
        using (var alert = new AlertDialog.Builder(this))
        {
            alert.SetTitle("Confirm Exit");
            alert.SetMessage("Are you sure you want to exit?");
            alert.SetPositiveButton("Yes", (sender, args) => { FinishAffinity(); }); // inform Android that we are done with the activity
            alert.SetNegativeButton("No", (sender, args) => {}); // do nothing

            var dialog = alert.Create();
            dialog.Show();
        }
        return;
    }
    base.OnBackPressed();
}

// Confirm with toast + back button
bool _isBackPressed = false;
public override void OnBackPressed()
{
    var app = (ConfirmExitMasterDetail.App)App.Current;
    if (app.PromptToConfirmExit)
    {
        if (_isBackPressed)
        {
            FinishAffinity(); // inform Android that we are done with the activity
            return;
        }

        _isBackPressed = true;
        Toast.MakeText(this, "Press back again to exit", ToastLength.Short).Show();

        // Disable back to exit after 2 seconds.
        new Handler().PostDelayed(() => { _isBackPressed = false; }, 2000);
        return;
    }
    base.OnBackPressed();
}