[RESOLVED] Why does my bitmap become all black when saved to file?

[Elrond]

I am trying to create a bitmap. This works well as I can obtain the correct result on the screen. But when I am trying to save the bitmap to a file, it become just a black rectangle.

I have tried a lot of things, multiple functions to save bitmap taken from different places, and always obtain the same result.

Here is a simplified version of the code, which produces the same result. It can simply be called in the simplest project that uses MFC. If I put a break point just before calling the write function, I can see my picture on the screen, as expected. But the bitmap is black in the file.

void CreateDrawing()
{
	CWnd wnd;
	CRect rect (0, 0, 100, 100);
	wnd.CreateEx(NULL, NULL, NULL,WS_POPUP |  WS_CHILD | WS_CLIPSIBLINGS, rect, NULL, NULL, NULL);

	CClientDC paintdc(&wnd);
	paintdc.SetBkMode(TRANSPARENT);

	CRect rectWnd = rect;
	CPoint ptToolTipLeft = rectWnd.TopLeft();
	SetWindowPos(wnd.m_hWnd, HWND_TOPMOST,ptToolTipLeft.x+1, ptToolTipLeft.y+1, rectWnd.Width(), rectWnd.Height(),SWP_SHOWWINDOW|SWP_NOOWNERZORDER|SWP_NOACTIVATE);

	CBrush m_brush(RGB(255,255,255));
	CPen m_pen(PS_SOLID, 0, COLORREF(RGB(0, 0, 0)));
	CBrush* pOldBrush = paintdc.SelectObject(&m_brush);
	CPen* pOldPen = paintdc.SelectObject(&m_pen);
	paintdc.Rectangle(0,0,rect.Width(),rect.Height());
	paintdc.MoveTo(0, 0);
	paintdc.LineTo(100, 100);


	HBITMAP hbmp = CreateCompatibleBitmap(paintdc.m_hDC, rect.Width(),rect.Height());
	CBitmap bmp;
	bmp.Attach(hbmp);

	WriteToFile(paintdc, hbmp);

	paintdc.SelectObject(pOldBrush);
	paintdc.SelectObject(pOldPen);
}


bool WriteToFile(CDC& hMemDC, HBITMAP hBitmap)
{
	CString strFileName = "c:\\test\\test.bmp";
	int   wBitCount = 32;

	BITMAP   Bitmap;  

	CPalette pal;
	pal.Attach(::GetStockObject(DEFAULT_PALETTE));	
	hMemDC.SelectPalette(&pal, TRUE);

	GetObject(hBitmap, sizeof(BITMAP), (LPSTR)&Bitmap); 

	BITMAPINFOHEADER   bi;
	bi.biSize = sizeof(BITMAPINFOHEADER);    
	bi.biWidth = Bitmap.bmWidth;    
	bi.biHeight = Bitmap.bmHeight;  
	bi.biPlanes = 1;    
	bi.biBitCount = wBitCount;    
	bi.biCompression = BI_RGB;    
	bi.biSizeImage = 0;  
	bi.biXPelsPerMeter = 0;    
	bi.biYPelsPerMeter = 0;    
	bi.biClrUsed = 0;    
	bi.biClrImportant = 0; 

	DWORD dwBmBitsSize = ((Bitmap.bmWidth * wBitCount + 31) / 32) * 4 * Bitmap.bmHeight; 

	HANDLE hDib = GlobalAlloc(GHND, dwBmBitsSize+sizeof(BITMAPINFOHEADER)); 
	LPBITMAPINFOHEADER lpbi = (LPBITMAPINFOHEADER)GlobalLock(hDib);    
	*lpbi = bi;

	GetDIBits(hMemDC.m_hDC, hBitmap, 0, (UINT)Bitmap.bmHeight,  
		(LPSTR)lpbi + sizeof(BITMAPINFOHEADER), (BITMAPINFO *)lpbi, DIB_RGB_COLORS);    


	DWORD dwDIBSize   =   sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + dwBmBitsSize;

	BITMAPFILEHEADER   bmfHdr;    
	bmfHdr.bfType = 'B'+('M' << 8);
	bmfHdr.bfSize = dwDIBSize;
	bmfHdr.bfReserved1 = 0;
	bmfHdr.bfReserved2 = 0;
	bmfHdr.bfOffBits = (DWORD)sizeof(BITMAPFILEHEADER) + (DWORD)sizeof(BITMAPINFOHEADER);    

	DWORD dwWritten;

	HANDLE fh = CreateFile(strFileName,GENERIC_WRITE,0,NULL,CREATE_ALWAYS,
			FILE_ATTRIBUTE_NORMAL|FILE_FLAG_SEQUENTIAL_SCAN,NULL);   
	ASSERT(fh != INVALID_HANDLE_VALUE);
	WriteFile(fh,   (LPSTR)&bmfHdr,   sizeof(BITMAPFILEHEADER),   &dwWritten,   NULL);    
	WriteFile(fh, (LPSTR)lpbi, dwDIBSize, &dwWritten, NULL);    
	GlobalUnlock(hDib);    
	GlobalFree(hDib);    
	CloseHandle(fh);
	return true;
}

I am probably missing something simple, but could not figure what.