Confirm while exit

ExitConfirm.cs

protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
        {
            if (MessageBox.Show("Are you sure?", "Exit?", MessageBoxButton.OKCancel) == MessageBoxResult.OK)
            {
                while (NavigationService.BackStack != null & NavigationService.BackStack.Count() > 0)
                    NavigationService.RemoveBackEntry();
            }
            else
                e.Cancel = true;
            base.OnBackKeyPress(e);
        }