How can I capture the screen from a windows service?

Question

I am writing a remote desktop application and I want to take a screenshot of the log in screen, the current logged user desktop, the screensaver and the UAC screen.

As a desktop application, taking a screenshot of the user desktop works fine.

Now I am implementing a windows service so that I can be able to take a screenshot of the UAC screen, the log in screen and the screensaver. However whatever I tried so far results in a black image.

I am using QService for writting a cross platform windows service and it starts fine. This is where I have reached until now to take a screenshot from the windows service:

On the main function I open the WinSta0 window station and create a new desktop

#include <QCoreApplication>
#include "andamaservice.h"

int main(int argc, char *argv[])
{

    auto winsta = OpenWindowStation(L"WinSta0", true, GENERIC_ALL);
        qDebug("OpenWindowStation lasterror =%u", GetLastError());


        //SECURITY_ATTRIBUTES attributes = {sizeof(SECURITY_ATTRIBUTES), 0, true};
        //attributes.nLength = sizeof(SECURITY_ATTRIBUTES);
        //attributes.bInheritHandle = true;
        //hNewDesktop = CreateDesktop("NewDesktopName", NULL, NULL, 0 , GENERIC_ALL,  &stSecurityAttr);

    auto hwinsta2 = SetProcessWindowStation(winsta);
        qDebug("SetProcessWindowStation lasterror =%u", GetLastError());


    //auto desktop = OpenDesktop(L"default", 0, false, GENERIC_ALL);
    //qDebug("OpenDesktop lasterror = %u", GetLastError());


    // Create the desktop.
    auto desktop = CreateDesktop(L"newdesktop",
                               NULL,
                               NULL,
                               0,
                               GENERIC_ALL,
                               &attributes);
    qDebug("CreateDesktop lasterror =%u", GetLastError());


    SetThreadDesktop(desktop);
    //SwitchDesktop(desktop);
    qDebug("SetThreadDesktop lasterror =%u", GetLastError());


    AndamaService service(argc, argv);
    return service.exec();
}

then, at some other point in my code, I am taking the screenshot, but the result is a black image. This is the code I am using:

// get the device context of the screen
HDC hScreenDC = CreateDC(L"DISPLAY", NULL, NULL, NULL);
// and a device context to put it in
HDC hMemoryDC = CreateCompatibleDC(hScreenDC);

int x = GetDeviceCaps(hScreenDC, HORZRES);
int y = GetDeviceCaps(hScreenDC, VERTRES);

// maybe worth checking these are positive values
HBITMAP hBitmap = CreateCompatibleBitmap(hScreenDC, x, y);

// get a new bitmap
HBITMAP hOldBitmap = (HBITMAP)SelectObject(hMemoryDC, hBitmap);

BitBlt(hMemoryDC, 0, 0, x, y, hScreenDC, 0, 0, SRCCOPY);
hBitmap = (HBITMAP)SelectObject(hMemoryDC, hOldBitmap);

qimg = QtWin::fromHBITMAP(hBitmap, QtWin::HBitmapNoAlpha).toImage();

// clean up
DeleteDC(hMemoryDC);
DeleteDC(hScreenDC);

The complete application source code and executables are here: http://andama.org/source-code

How can I correctly get a screenshot of the desktop (or log in screen, or UAC screen) from a windows service?

Have you seen this [Qt screenshot example ?](https://doc.qt.io/qt-5/qtwidgets-desktop-screenshot-example.html) — Vladimir Bershov, Jan 25 '16 at 14:07
@VladimirBershov yes, I tried the same method but I get the same black screen when running as a service — Yiannis Mpourkelis, Jan 25 '16 at 14:10
I think these links will be useful: [**one**](http://stackoverflow.com/a/15375487/4149835) and [**two**](https://stackoverflow.com/questions/7454106/windows-service-couldnt-get-screenshot-in-windows-7) — Vladimir Bershov, Jan 25 '16 at 14:16
We did this in C#, but the API's should still apply: https://stackoverflow.com/a/45095509/125406 — Michael Silver, Jul 14 '17 at 05:49

score 0 · Answer 1 · edited Aug 15 '18 at 00:02

0

I don't think you want this: CreateDesktop, but instead want this:

_hDesktop = NativeMethods.OpenDesktop("Default", 0, true, Constants.MAXIMUM_ALLOWED);

[DllImport("User32.dll", SetLastError = true)]
public static extern IntPtr OpenDesktop(String lpszDesktop, int dwFlags, bool fInherit, int dwDesiredAccess);

edited Aug 15 '18 at 00:02

Stephen Rauch

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answered Aug 14 '18 at 23:29

Sean Roberts

1

How can I capture the screen from a windows service?

1 Answers1