private void btnRemoveAllSubject_Click(object sender, RoutedEventArgs e)
 {
     foreach (var item in SelectedSubjectList)
     {
         UnSelectedSubjectList = UnSelectedSubjectList.Add(item);
         SelectedSubjectList   = SelectedSubjectList.Remove(item);
     }
 }
        private void btnRemoveSelectedSingleSubject_Click(object sender, RoutedEventArgs e)
        {
            var selectedItems = gvClassSubjectSelected.SelectedItems;

            if (selectedItems == null)
            {
                return;
            }

            for (int i = 0; i < selectedItems.Count; i++)
            {
                Subject item = (Subject)selectedItems[i];
                UnSelectedSubjectList = UnSelectedSubjectList.Add(item);
                SelectedSubjectList   = SelectedSubjectList.Remove(item);
            }
        }