void khuyenmaiclick(object sender, RoutedEventArgs e)
 {
     frmkhuyenmai frm = new frmkhuyenmai();
     frm.Show();
 }
 private void ChildWindow_Closed(object sender, EventArgs e)
 {
     frmkhuyenmai frm = new frmkhuyenmai();           
     frm.Show();
 }