Windows change system resolution c++ - windows

Question

I am trying to change system resolution programatically. What I need to achieve is set system resolution to maximum value.
Below is code the code which change the screen resolution to maximum available value.
DEVMODE devmode = { 0 };
int i=0;
QList<int>widths;
QList<int>heights;
while (1) {
if(EnumDisplaySettings( NULL,i, &devmode )!=true)
break;
widths.append(devmode.dmPelsWidth);
heights.append(devmode.dmPelsHeight);
qDebug()<<devmode.dmPelsWidth<<" X "<<devmode.dmPelsHeight;
i++;
}
EnumDisplaySettings( NULL,ENUM_CURRENT_SETTINGS, &devmode );
devmode.dmSize = sizeof(DEVMODE);
devmode.dmPelsWidth = widths.at(widths.size()-1); //take last item maximum value
devmode.dmPelsHeight = heights.at(widths.size()-1); //take last item maximum value
long result = ChangeDisplaySettings(&devmode, DM_PELSWIDTH || DM_PELSHEIGHT);
And it works fine when,
When the system resolution is maximum(1920x1080) I change to the same value.
When the system resolution is higher and I am changed to lower value.
But it doesn't work when I am changing from lower value to higher, like current resolution is 1600X900 and when I am changing to 1920x1080 then it wont work.
And I am getting -2 as return value and which stand for The graphics mode is not supported on the doc.

Answer 1

There are 2 issues with the way you're calling the function.
First, there is a typo. DM_PELSWIDTH || DM_PELSHEIGHT evaluates to 1, not the combination of the two flags. You probably meant DM_PELSWIDTH | DM_PELSHEIGHT
Second, those are not the flags you should be passing to ChangeDisplaySettings. Here's the correct way to call the function:
DEVMODE desiredMode = { 0 };
desiredMode.dmSize = sizeof(DEVMODE);
desiredMode.dmPelsWidth = 1920;
desiredMode.dmPelsHeight = 1080;
desiredMode.dmFields = DM_PELSHEIGHT | DM_PELSWIDTH;
LONG res = ChangeDisplaySettings(&desiredMode, CDS_UPDATEREGISTRY | CDS_GLOBAL | CDS_RESET);
The dmFields member of the DEVMODE structure is where you tell the system which fields you want to change. The flags on ChangeDisplaySettings specify how those settings are applied. The reason why the call with DM_PELSWIDTH || DM_PELSHEIGHT didn't fail with DISP_CHANGE_BADFLAGS is that 1 is the value of the CDS_UPDATEREGISTRY flag.
As a side note: if you are just temporarily changing resolution for your application (like games do) then pass the flag CDS_FULLSCREEN by itself and it will revert the settings when your application exits. The combination of flags in the example above, sets the settings for all users, stores it in the registry, and applies the change immediately.