There are various ways of getting a monitor. You can get the monitor from a point, from a rectangle, or from a window. But how do you get the primary monitor?

The primary monitor is defined to be the one which has (0, 0) as its origin. Therefore, one solution is

HMONITOR GetPrimaryMonitor()
{
 POINT ptZero = { 0, 0 };
 return MonitorFromPoint(ptZero,
                         MONITOR_DEFAULTTOPRIMARY);
}

The desktop window by convention is deemed to reside primarily on the primary monitor, so you could also use this:

HMONITOR GetPrimaryMonitor()
{
 return MonitorFromWindow(GetDesktopWindow(),
                          MONITOR_DEFAULTTOPRIMARY);
}

Or you could just pass the null window handle. This is technically an illegal parameter, but by specifying MONITOR_DEFAULT­TO­PRIMARY, you are saying, “If anything goes wrong, give me the primary monitor.”

HMONITOR GetPrimaryMonitor()
{
 return MonitorFromWindow(nullptr,
                          MONITOR_DEFAULTTOPRIMARY);
}

In this case, we are intentionally going astray because we want to kick in the MONITOR_DEFAULT­TO­PRIMARY behavior.