ChangeDisplaySettings only seems to work for primary monitor (returns DISP_CHANGE_BADMODE)

Question

I have two displays in my system: my laptop display and an external monitor. Independent from which is the primary screen, I would like to change the screen rotation. I started from the examples in http://msdn.microsoft.com/en-us/library/ms812499.aspx and this is my simple test code:

DWORD displayNumber = 1;

DISPLAY_DEVICE dd;
ZeroMemory(&dd, sizeof(dd));
dd.cb = sizeof(dd);

DEVMODE dm;
ZeroMemory(&dm, sizeof(dm));
dm.dmSize = sizeof(dm);

if (0 != EnumDisplayDevices(NULL, displayNumber , &dd, EDD_GET_DEVICE_INTERFACE_NAME))
{
    if (0 != EnumDisplaySettings(dd.DeviceName, ENUM_CURRENT_SETTINGS, &dm))
    {
        // swap height and width
        DWORD dwTemp = dm.dmPelsHeight;
        dm.dmPelsHeight = dm.dmPelsWidth;
        dm.dmPelsWidth = dwTemp;

        // determine new orientaion
        switch (dm.dmDisplayOrientation)
        {
        case DMDO_DEFAULT:
            dm.dmDisplayOrientation = DMDO_270;
            break;
        case DMDO_270:
            dm.dmDisplayOrientation = DMDO_180;
            break;
        case DMDO_180:
            dm.dmDisplayOrientation = DMDO_90;
            break;
        case DMDO_90:
            dm.dmDisplayOrientation = DMDO_DEFAULT;
            break;
        default:
            break;
        }
        long lRet = ChangeDisplaySettings(&dm, 0);
        if (DISP_CHANGE_SUCCESSFUL != lRet)
        {
            switch (lRet)
            {
                case DISP_CHANGE_BADDUALVIEW:
                    MessageBox(0, _T("DISP_CHANGE_BADDUALVIEW"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                case DISP_CHANGE_BADFLAGS:
                    MessageBox(0, _T("DISP_CHANGE_BADFLAGS"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                case DISP_CHANGE_BADMODE:
                    MessageBox(0, _T("DISP_CHANGE_BADMODE"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                case DISP_CHANGE_BADPARAM:
                    MessageBox(0, _T("DISP_CHANGE_BADPARAM"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                case DISP_CHANGE_FAILED:
                    MessageBox(0, _T("DISP_CHANGE_FAILED"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                case DISP_CHANGE_NOTUPDATED:
                    MessageBox(0, _T("DISP_CHANGE_NOTUPDATED"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                case DISP_CHANGE_RESTART:
                    MessageBox(0, _T("DISP_CHANGE_RESTART"), _T("ChangeDisplaySettings failed"), 0);
                    break;
                default:
                    MessageBox(0, _T("default"), _T("ChangeDisplaySettings failed"), 0);
                    break;
            }
        }
    }
}

Right now, the first display is the laptop display (and also the primary display) and the second display is external monitor.

If I execute the code above with displayNumber = 0, everything works as expected: The laptop screen rotates and the main screen stays as it is.

However, if I set displayNumber = 1, ChangeDisplaySettings returns DISP_CHANGE_BADMODE. It seems the rotation can only be changed for the primary display that way.

Contrary, if I set the external monitor to be the primary display, things work as expected for displayNumber = 1. However, with displayNumber = 0, the settings on the external monitor get all messed up while the laptop display (first display) rotates, as expected.

What's going on here?

Answer 1

It seems I need to use ChangeDisplaySettingsEx instead and explicitely specify the display device. I found a sample code on https://gist.github.com/umq/986635 that works.

Instead of

long lRet = ChangeDisplaySettings(&dm, 0);

write:

long lRet = ChangeDisplaySettingsEx(dd.DeviceName, &dm, NULL, 0, NULL);

Done.