Draw over Dialog Margins in an MFC Dialog - c++

Question

I want a dialog to be borderless and yet have a dialog shadow. I came across this solution Borderless Window Using Areo Snap, Shadow, Minimize Animation, and Shake which uses a workaround by making the Dialog having a Margin of 1 px and extending the Client Area to it.
MARGINS borderless = { 1, 1, 1, 1 };
DwmExtendFrameInfoClientArea(this->GetSafeHwnd(), &borderless);
The post mentioned that the Client Area is literally being extended and Transparent drawing makes the Dialog edges of 1px each visible again.
Now this is exactly what happened, when I tried to paint a Solid Rectangle onto the whole dialog:
// getting the client area
CRect clientRect;
GetClientRect(&clientRect);
// expanding it to the new margins
clientRect.left -= 1;
clientRect.top -= 1;
clientRect.right += 2;
clientRect.bottom += 2;
// set the Device Context to draw non transparent and with a black background
pDC->SetBkMode(OPAQUE);
pDC->SetBkColor(RGB(0, 0, 0));
// finally draw a rectangle to it
CBrush brush_back_ground(RGB(0, 0, 0));
pDC->FillRect(clientRect, &brush_back_ground);
But the dialog is still drawn with its margins:
How would it be possible to draw something stretched on the margins? Later I'm going to use pictures as dialog Background which should be drawn on the margins aswell.

Answer 1

Thanks to Hans Passant for his comment. The solution is to use GDI+ drawing instead of GDI drawing
// making a Gdi+ graphics object from my CDC*
Graphics g(*pDC);
// getting the client area
CRect clientRect;
GetClientRect(&clientRect);
// making a Gdi+ rect
Rect bkRect(0,0,clientRect.Width(), clientRect.Height());
// making a pen for the Rect Drawing
Pen bkPen(Color(255,0,0,0));
// draw the rectangle over the full dialog
g.DrawRectangle(&bkPen, bkRect);