e.Handle in Date picker while lostfocus

private void datePicker1_LostFocus(object sender, RoutedEventArgs e)
  {

    DateTime StartDate = new DateTime(2011, 4, 1);
    DateTime EndDate = new DateTime(2011, 4, 1);

    DatePicker dDate = (DatePicker)sender;
    //DateTime ddate = dDate.DisplayDate;

    if ((dDate.DisplayDate >= StartDate) && dDate == null)      // && (StartDate <= dDate.DisplayDate))
    {
        MessageBox.Show("good");
    }
    else
    {
        MessageBox.Show("bad");

        e.Handled = true;
    }

}