Problem with on_expose_event

[Robert Hildebrandt <roberts_katz gmx de>]

Hi I want to change the look of a button. So I derived Gtk::Button and 
overloaded the on expose event. But this Code doesn't show any change:

bool CloseButton::on_expose_event(GdkEventExpose* ee)
{
 Button::on_expose_event(ee);

  r_assert(get_window());

 Cairo::RefPtr<Cairo::Context> cc  = get_window()->create_cairo_context();
 g_assert(cc);

 cc->rectangle(0, 0, 18, 18);
 cc->set_source_rgb(1, 0, 1);
 cc->fill();

 return true;
}

Thangs in advance
Robert