// handle the Click event on the _chkFreeze CheckBox to
// freeze/unfreeze rows/columns
void _chkFreeze_Click(object sender, RoutedEventArgs e)
{
  if (_chkFreeze.IsChecked.Value)
  {
    // freeze rows and columns above and to the left of the cursor
    _flex.Rows.Frozen = _flex.Selection.Row;
    _flex.Columns.Frozen = _flex.Selection.Column;
  }
  else
  {
    // unfreeze rows and columns
    _flex.Rows.Frozen = 0;
    _flex.Columns.Frozen = 0;
  }
}