How to get the scale factor to obtain the real screen resolution?

Question

I'm trying to get the screen resolution scale factor to obtain the correct screen resolution. I tried different scale factor combinations but the 'dsf' variable is always set to 100. How can I fix this?

RECT System::GetScreenResolution(HWND hwnd)
{
    //get resolution scale factor
    HMONITOR hmonitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTOPRIMARY);
    DEVICE_SCALE_FACTOR dsf;
    GetScaleFactorForMonitor(hmonitor, &dsf);

    //get and calculate real resolution
    HDC hdc = GetDC(hwnd);
    
    RECT r{ 0, 0, GetDeviceCaps(hdc, HORZRES) * dsf / 100, GetDeviceCaps(hdc, VERTRES) * dsf / 100 };

    ReleaseDC(hwnd, hdc);

    return r;
}

I build the program under Windows 10 64bit using Visual Studio 2019 and I'm using multiple monitor with extended display. Thanks in advance.

Answer 1

For GetScaleFactorForMonitor() to work accurately under Windows 10, you need to have a manifest with a corresponding supported OS id...

Note that, with dpiAware = true in the manifest, GetDeviceCaps(hdc, HORZRES) will give valid result on application start but erroneous result after a dynamic scale change – so read it on start and keep this value.

   <!-- Compatibility section for Program Compatibility Assistant (PCA) -->
   <compatibility xmlns='urn:schemas-microsoft-com:compatibility.v1'>
     <application>
       <supportedOS Id='{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}'/>
     </application>
   </compatibility>