private void btn_Back_Click(object sender, RoutedEventArgs e)
{
PurchasePage pp = new PurchasePage();
this.NavigationService.Navigate(pp);
clearList();
}
private void CommandBinding_Executed_1(object sender, ExecutedRoutedEventArgs e)
{
PurchasePage pp = new PurchasePage();
frame.NavigationService.Navigate(pp);
}