Is there a way to detect if a monitor is plugged in? - c++

Question

I have a custom application written in C++ that controls the resolution and other settings on a monitor connected to an embedded system. Sometimes the system is booted headless and run via VNC, but can have a monitor plugged in later (post boot). If that happens he monitor is fed no video until the monitor is enabled. I have found calling "displayswitch /clone" brings the monitor up, but I need to know when the monitor is connected. I have a timer that runs every 5 seconds and looks for the monitor, but I need some API call that can tell me if the monitor is connected.
Here is a bit of psudocode to describe what I'm after (what is executed when the timer expires every 5 seconds).
if(If monitor connected)
{
ShellExecute("displayswitch.exe /clone);
}else
{
//Do Nothing
}
I have tried GetSystemMetrics(SM_CMONITORS) to return the number of monitors, but it returns 1 if the monitor is connected or not. Any other ideas?
Thanks!

Answer 1

Try the following code
BOOL IsDisplayConnected(int displayIndex = 0)
{
DISPLAY_DEVICE device;
device.cb = sizeof(DISPLAY_DEVICE);
return EnumDisplayDevices(NULL, displayIndex, &device, 0);
}
This will return true if Windows identifies a display device with index (AKA identity) 0 (this is what the display control panel uses internally). Otherwise, it will return false false. So by checking the first possible index (which I marked as the default argument), you can find out whether any display device is connected (or at least identified by Windows, which is essentially what you're looking for).

Answer 2

Seems that there is some kind of "default monitor" even if no real monitor is connected.
The function below works for me (tested on a Intel NUC and a Surface 5 tablet).
The idea is to get the device id and check if it contains the string "default_monitor".
bool hasMonitor()
{
// Check if we have a monitor
bool has = false;
// Iterate over all displays and check if we have a valid one.
// If the device ID contains the string default_monitor no monitor is attached.
DISPLAY_DEVICE dd;
dd.cb = sizeof(dd);
int deviceIndex = 0;
while (EnumDisplayDevices(0, deviceIndex, &dd, 0))
{
std::wstring deviceName = dd.DeviceName;
int monitorIndex = 0;
while (EnumDisplayDevices(deviceName.c_str(), monitorIndex, &dd, 0))
{
size_t len = _tcslen(dd.DeviceID);
for (size_t i = 0; i < len; ++i)
dd.DeviceID[i] = _totlower(dd.DeviceID[i]);
has = has || (len > 10 && _tcsstr(dd.DeviceID, L"default_monitor") == nullptr);
++monitorIndex;
}
++deviceIndex;
}
return has;
}