How to use MONITORINFOEX with GetMonitorInfoA?

[SageSudo]

Hi,

According to the Microsoft documentation for GetMonitorInfo, the function's second parameter can be either a MONITORINFO or MONITORINFOEX, however, in the windows-rs documentation for GetMonitorInfoA, the function only accepts MONITORINFO as a second parameter.

Is there something I have to do to make MONITORINFOEX work with GetMonitorInfoA?

This is how I've went about trying GetMonitorInfoA:

use windows::Win32::{
    Foundation::POINT,
    Graphics::Gdi::{
        GetMonitorInfoA, MonitorFromPoint, MONITORINFO, MONITORINFOEXA, MONITOR_DEFAULTTONULL,
    },
    UI::WindowsAndMessaging::GetCursorPos,
};

fn main() {
    unsafe {
        let mut point = POINT::default();
        GetCursorPos(&mut point);

        let hmonitor = MonitorFromPoint(point, MONITOR_DEFAULTTONULL);

        let mut monitor_info = MONITORINFO {
            cbSize: std::mem::size_of::<MONITORINFOEXA>() as u32,
            ..MONITORINFO::default()
        };

        let mut monitor_info_ex = MONITORINFOEXA {
            monitorInfo: monitor_info,
            ..MONITORINFOEXA::default()
        };

        GetMonitorInfoA(hmonitor, &mut monitor_info_ex); //<-- Not possible.
    }
}

[riverar]

Hi @SageSudo, pass in monitor_info_ex.monitorInfo. It starts at the same offset as the extended structure and cleanly gets you around this funky API.

[SageSudo]

Hi @riverar, thank you for answering!

I did give that a try when I was experimenting with the function previously but I realized that the szDevice (display monitor name) field in MONITORINFOEXA won't get populated with the monitor name.

When I tried println!("monitor_info_ex.szDevice {:?}", monitor_info_ex.szDevice);, I get the below value regardless of monitor:

monitor_info_ex.szDevice [CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0), CHAR(0)]

Is that expected because I only passed in monitor_info_ex.monitorInfo or should I have received a value in the szDevice field?

[kennykerr]

That's likely because you didn't initialize the MONITORINFOEXA with its size. It is the cbSize field that tells the GetMonitorInfoA function which structure you actually passed in. Here's a complete example:

use windows::{Win32::Foundation::*, Win32::Graphics::Gdi::*};

fn main() {
    unsafe {
        EnumDisplayMonitors(None, std::ptr::null(), Some(callback), LPARAM(0));
    }
}

extern "system" fn callback(handle: HMONITOR, _: HDC, _: *mut RECT, _: LPARAM) -> BOOL {
    unsafe {
        let mut info = MONITORINFOEXA::default();
        info.monitorInfo.cbSize = std::mem::size_of::<MONITORINFOEXA>() as _;
        GetMonitorInfoA(handle, &mut info.monitorInfo);
        let slice = std::slice::from_raw_parts(info.szDevice.as_ptr() as _, info.szDevice.len());
        println!("{}", std::str::from_utf8_unchecked(slice));
        BOOL(1)
    }
}

And here's what I get:

\\.\DISPLAY1
\\.\DISPLAY2

Check the docs for GetMonitorInfoA for more information:

https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getmonitorinfoa

[SageSudo]

Yes, I must have done something wrong, I rewrote what I had and followed both your suggestions and that worked.

Thank you for the example Kenny, and thank you both for the help.