How do I save GDI+ Graphics / HDC to a file? - c++

Question

I am unable to save a GDI+ Graphics object, which is derived from a Device Context HDC, to a file.
What works: I am able to save a GDI+ Graphics derived from a Bitmap. Sample code (Win32):
Color color(255, 0, 0);
Pen pen(color, 2.0f);
CLSID pngClsid;
if(GetEncoderClsid(L"image/bmp", &pngClsid) < 0) // calls GetImageEncoders()
return;
// Graphics from Bitmap - works OK
Bitmap bitmap(300, 300, PixelFormat24bppRGB); // create Bitmap first
Graphics *graphics = new Graphics(&bitmap); // create Graphics second
graphics->Clear(Color(255, 255, 255, 255));
Status stat = graphics->DrawEllipse(&pen, 50, 50, 100, 100);
assert(stat == Ok);
stat = bitmap.Save(L"C:\\temp\\test1.bmp", &pngClsid, NULL);
assert(stat == Ok);
delete graphics;
Result:
What fails: If the Graphics object is derived from an HDC, I get a black rectangle. Regardless of whether I create the Bitmap at point [1], [2] or [3], I always get a black rectangle. Code:
CLSID pngClsid;
if(GetEncoderClsid(L"image/bmp", &pngClsid) < 0) // calls GetImageEncoders()
return;
// Graphics from HDC - fails
HDC hdc = GetDC(NULL);
Graphics *graphicsDC = new Graphics(hdc); // create Graphics first
graphicsDC->SetPageUnit(UnitPixel);
//Bitmap bitmapDC(300, 300, graphicsDC); // [1] create Bitmap second. Black rectangle if called here
graphicsDC->Clear(Color(255, 255, 255, 255));
//Bitmap bitmapDC(300, 300, graphicsDC); // [2] black rectangle if called here
HPEN penGDI = CreatePen(PS_SOLID, 3, RGB(0, 255, 0)); // old school GDI
HPEN oldPen = (HPEN)SelectObject(hdc, penGDI);
Ellipse(hdc, 50, 50, 150, 150);
DeleteObject(SelectObject(hdc, oldPen));
Bitmap bitmapDC(300, 300, graphicsDC); // [3] black rectangle if called here
Status stat = bitmapDC.Save(L"C:\\temp\\test2.bmp", &pngClsid, NULL);
assert(stat == Ok);
delete graphicsDC;
ReleaseDC(NULL, hdc);
Result:
Why I need this: I am converting code that contains thousands of calls to the GDI API. I want to start using GDI+ gradually without converting all GDI calls to GDI+ at once. I have mixed GDI/GDI+ successfully in other cases, for instance when creating gradients. The only difference is that in the other cases, I wasn't trying to save to a file.
I can't use CImage because it doesn't have groovy anti-aliasing.
So, how can I save a Graphic as image when starting with an HDC?