I want to obtain a monitor handle (HMONITOR
) that can be used with the Windows multi-monitor APIs for a specific monitor attached to the system by index. For example, say I have three monitors attached to my system and forming part of my desktop; I want to get a handle to monitor 3.
I already know how to get the device name for a specific monitor by index by calling the EnumDisplayDevices
function. For example:
HMONITOR MonitorFromIndex(int index /* (zero-indexed) */)
{
DISPLAY_DEVICE dd;
dd.cb = sizeof(dd);
if (EnumDisplayDevices(NULL, index, &dd, 0) != FALSE)
{
// We found a match; make sure that it's part of the desktop.
if ((dd.StateFlags & DISPLAY_DEVICE_ATTACHED_TO_DESKTOP) == DISPLAY_DEVICE_ATTACHED_TO_DESKTOP)
{
// Yup. Now we've got the name of the device:
std::cout << dd.DeviceName << std::endl;
// But how do I obtain an HMONITOR for this device?
// ...
}
}
return NULL; // indicate failure
}
In the code above, we've found the name of the desired device (dd.DeviceName
). I can use this name to create a DC for that monitor by calling CreateDC
:
HDC hDC = CreateDC(dd.DeviceName, dd.DeviceName, NULL, NULL);
And I can obtain information about that monitor by calling EnumDisplaySettings
:
DEVMODE dm;
dm.dmSize = sizeof(dm);
dm.dmDriverExtra = 0;
if (EnumDisplaySettings(dd.DeviceName, ENUM_CURRENT_SETTINGS, &dm) != FALSE)
{
std::cout << "The monitor supports " << dm.dmBitsPerPel << " bits per pixel." << std::endl;
}
Which is all great, but I want a handle to that monitor. How can I get it?
I tried to call EnumDisplayMonitors
, passing a handle to the device context that I created using CreateDC
, hoping to get a handle to the monitor passed to the callback function, but no such luck. The callback function was never called, and EnumDisplayMonitors
returned FALSE
(without setting an error code):
struct FoundMatch
{
BOOL found;
HMONITOR hMonitor;
};
BOOL CALLBACK MonitorEnumProc(HMONITOR hMonitor, HDC, LPRECT, LPARAM dwData)
{
FoundMatch* pfm = reinterpret_cast<FoundMatch*>(dwData);
pfm->found = TRUE;
pfm->hMonitor = hMonitor;
return FALSE; // stop enumerating
}
// elsewhere, after getting the device name and using it to create a DC
FoundMatch fm;
fm.found = FALSE;
fm.hMonitor = NULL;
BOOL result = EnumDisplayMonitors(hDC, NULL, MonitorEnumProc, reinterpret_cast<LPARAM>(&fm));
Sorry for such a late reply but maybe someone can find this useful.
The multi-monitor API is really minimalist to say the least. Once you've got your dd.DeviceName
, it appears you have to go through EnumDisplayMonitors()
enumeration until you find a match of dd.DeviceName
against MONITORINFOEX.szDevice
.
The MONITORINFOEX
structure can be obtained by calling GetMonitorInfo()
.
Here is a non-compilable C++11 pseudo code:
struct DataBag
{
HMONITOR hmon;
TCHAR* devname;
} bag;
bag.hmon = NULL;
bag.devname = &dd.DeviceName;
BOOL bRes = EnumDisplayMonitors(
NULL, NULL,
[](HMONITOR hMonitor, HDC hDC, LPRECT rc, LPARAM data) -> BOOL {
auto& bag = *reinterpret_cast<DataBag*>(data);
MONITORINFOEX mi;
mi.cbSize = sizeof(mi);
if (/* match bag.devname against mi.szDevice */ && GetMonitorInfo(hMonitor, &mi))
{
bag.hmon = hMonitor;
return FALSE;
}
return TRUE;
},
reinterpret_cast<LPARAM>(&bag));
if (bRes && bag.hmon)
{
// Monitor found!
}
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