How can I take a screenshot in a windows application?

How can I take a screenshot of the current screen using Win32?

HDC hScreenDC = GetDC(nullptr); // CreateDC("DISPLAY",nullptr,nullptr,nullptr);
HDC hMemoryDC = CreateCompatibleDC(hScreenDC);
int width = GetDeviceCaps(hScreenDC,HORZRES);
int height = GetDeviceCaps(hScreenDC,VERTRES);
HBITMAP hBitmap = CreateCompatibleBitmap(hScreenDC,width,height);
HBITMAP hOldBitmap = static_cast<HBITMAP>(SelectObject(hMemoryDC,hBitmap));
BitBlt(hMemoryDC,0,0,width,height,hScreenDC,0,0,SRCCOPY);
hBitmap = static_cast<HBITMAP>(SelectObject(hMemoryDC,hOldBitmap));
DeleteDC(hMemoryDC);
DeleteDC(hScreenDC);

1. Use GetDC(NULL); to get a DC for the entire screen.
2. Use CreateCompatibleDC to create a DC compatible with the screen DC.
3. Use CreateCompatibleBitmap to create a bitmap compatible with the screen DC to hold the result.
4. Use SelectObject to select the compatible bitmap into the compatible DC.
5. Use BitBlt to copy from the screen DC to the compatible DC.
6. Use SelectObject to deselect the compatible bitmap from the compatible DC.
7. Use DeleteDC to delete the compatible DC.

When you create the compatible bitmap, you want it compatible with the screen DC, not the compatible DC.

For example:

HDC dcScreen = GetDC(0);
HDC dcTarget = CreateCompatibleDC(dcScreen);
HBITMAP bmpTarget = CreateCompatibleBitmap(dcScreen);
HGDIOBJ oldBmp = SelectObject(dcTarget, bmpTarget);
BitBlt(dcTarget, 0, 0, cx, cy, dcDesktop, x, y, SRCCOPY | CAPTUREBLT);
SelectObject(dcTarget, oldBmp);
DeleteDC(dcTarget);
ReleaseDC(dcScreen);

The other important part is to get the size, and location, of the entire virtual screen:

int x  = GetSystemMetrics(SM_XVIRTUALSCREEN);  //left (e.g. -1024)
int y  = GetSystemMetrics(SM_YVIRTUALSCREEN);  //top (e.g. -34)
int cx = GetSystemMetrics(SM_CXVIRTUALSCREEN); //entire width (e.g. 2704)
int cy = GetSystemMetrics(SM_CYVIRTUALSCREEN); //entire height (e.g. 1050)

void GetScreenShot(void)
{
	int x1, y1, x2, y2, w, h;

	// get screen dimensions
	x1	= GetSystemMetrics(SM_XVIRTUALSCREEN);
	y1	= GetSystemMetrics(SM_YVIRTUALSCREEN);
	x2	= GetSystemMetrics(SM_CXVIRTUALSCREEN);
	y2	= GetSystemMetrics(SM_CYVIRTUALSCREEN);
	w	= x2 - x1;
	h	= y2 - y1;

	// copy screen to bitmap
	HDC		hScreen = GetDC(NULL);
	HDC		hDC		= CreateCompatibleDC(hScreen);
	HBITMAP hBitmap = CreateCompatibleBitmap(hScreen, w, h);
	HGDIOBJ old_obj	= SelectObject(hDC, hBitmap);
	BOOL	bRet	= BitBlt(hDC, 0, 0, w, h, hScreen, x1, y1, SRCCOPY);
	
	// save bitmap to clipboard
	OpenClipboard(NULL);
	EmptyClipboard();
	SetClipboardData(CF_BITMAP, hBitmap);
	CloseClipboard();	
		
	// clean up
	SelectObject(hDC, old_obj);
	DeleteDC(hDC);
	ReleaseDC(NULL, hScreen);
	DeleteObject(hBitmap);
}

Full code for saving a raw 24-bit lossless bitmap of all monitors at the current window station using Windows API:

BOOL WINAPI SaveBitmap(WCHAR *wPath)
{
	BITMAPFILEHEADER bfHeader;
	BITMAPINFOHEADER biHeader;
	BITMAPINFO bInfo;
	HGDIOBJ hTempBitmap;
	HBITMAP hBitmap;
	BITMAP bAllDesktops;
	HDC hDC, hMemDC;
	LONG lWidth, lHeight;
	BYTE *bBits = NULL;
	HANDLE hHeap = GetProcessHeap();
	DWORD cbBits, dwWritten = 0;
	HANDLE hFile;
    INT x = GetSystemMetrics(SM_XVIRTUALSCREEN);
    INT y = GetSystemMetrics(SM_YVIRTUALSCREEN);

	ZeroMemory(&bfHeader, sizeof(BITMAPFILEHEADER));
	ZeroMemory(&biHeader, sizeof(BITMAPINFOHEADER));
	ZeroMemory(&bInfo, sizeof(BITMAPINFO));
	ZeroMemory(&bAllDesktops, sizeof(BITMAP));

	hDC = GetDC(NULL);
	hTempBitmap = GetCurrentObject(hDC, OBJ_BITMAP);
	GetObjectW(hTempBitmap, sizeof(BITMAP), &bAllDesktops);

	lWidth = bAllDesktops.bmWidth;
	lHeight = bAllDesktops.bmHeight;

	DeleteObject(hTempBitmap);

	bfHeader.bfType = (WORD)('B' | ('M' << 8));
	bfHeader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
	biHeader.biSize = sizeof(BITMAPINFOHEADER);
	biHeader.biBitCount = 24;
	biHeader.biCompression = BI_RGB;
	biHeader.biPlanes = 1;
	biHeader.biWidth = lWidth;
	biHeader.biHeight = lHeight;

	bInfo.bmiHeader = biHeader;

	cbBits = (((24 * lWidth + 31)&~31) / 8) * lHeight;

	hMemDC = CreateCompatibleDC(hDC);
	hBitmap = CreateDIBSection(hDC, &bInfo, DIB_RGB_COLORS, (VOID **)&bBits, NULL, 0);
	SelectObject(hMemDC, hBitmap);
	BitBlt(hMemDC, 0, 0, lWidth, lHeight, hDC, x, y, SRCCOPY);

	
	hFile = CreateFileW(wPath, GENERIC_WRITE | GENERIC_READ, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
	WriteFile(hFile, &bfHeader, sizeof(BITMAPFILEHEADER), &dwWritten, NULL);
	WriteFile(hFile, &biHeader, sizeof(BITMAPINFOHEADER), &dwWritten, NULL);
	WriteFile(hFile, bBits, cbBits, &dwWritten, NULL);

	CloseHandle(hFile);

	DeleteDC(hMemDC);
	ReleaseDC(NULL, hDC);
	DeleteObject(hBitmap);

	return TRUE;
}

There is a MSDN sample, http://msdn.microsoft.com/en-us/library/dd183402">Capturing an Image, for capturing an arbitrary HWND to a DC (you could try passing the output from GetDesktopWindow to this). But how well this will work under the new desktop compositor on Vista/Windows 7, I don't know.