Call me lazy:
BEGIN_EVENT_TABLE(MyWindow, wxWindow) EVT_PAINT (MyWindow::OnPaint) END_EVENT_TABLE() void MyWindow::OnPaint( wxPaintEvent &event ) { wxPaintDC dc(this); if (IsExposed( 0, 0, 100, 100)) { dc.SetPen( *wxBLACK_PEN ); dc.SetBrush( *wxRED_BRUSH ); dc.DrawRectangle( 0, 0, 100, 100 ); } }