How can I take a screenshot in a windows application?

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);

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);
}

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)