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MFC Example Multiple monitor support with GetSystemMetrics EnumDisplayMonitors and GetMonitorInfo


Sample Image - multimon.jpg


Awhile back I decided to write a screensaver application as a way to pick up MFC. After I got my first implementation up and running I shipped it off to a friend. Well the first thing he noted was a a glaring bug: he uses multiple monitors and my screen saver only showed up on one.

After some scouring of the internet and research into MSDN I was able to to get the screensaver running on multiple monitors by using some API calls the were introduced with Windows 98/2000. This project is a couple of small MFC classes that wrap the multiple-monitor API.

These classes can be safely used on Windows 95/NT4 as well. On those platforms, they just provide the properties of the one and only monitor.


Eveyrything you ever wanted to know about the API for multiple monitors is described in a very well written article by David Campbell in the June 1997 issue of MSJ.

The API itself is simple and straightforward. It has some new constants to pass to GetSystemMetrics and a couple methods to enumerate all of the monitors currently attached to the system and get the properties of a given monitor.

All of the types and functions are defined by the platform SDK in the file multimon.h . If you #define WINVER to be greater than 0x400 you don't need to link to multimon.h, as the API gets defined in windef.h for Windows 98 and later targeted builds. Multimon.h also provides some stub functions that allow calls to be safely made on Windows 95/NT4 machines. These wrappers interrogate the runtime OS and either make fall-through calls into the actual API or return the propeties of the one (and only) monitor on those older platforms.

The multiple monitor classes take care of including multimon.h correctly depending on the value of WINVER.

Using the code

CMointor is a basic MFC class that allows you to safely use the multiple-monitor API on any Win32 platform. 

There are three classes in this library:

CMonitors represents the collection of monitors currently attached to the system and wraps the EnumDisplayMonitors API function.

Collapse Copy Code
//CMonitors' interface
CMonitor GetMonitor( const int index ) const;
int GetCount() const; 

//returns the monitor closest to the specified item
static CMonitor GetNearestMonitor( const LPRECT lprc );
static CMonitor GetNearestMonitor( const POINT pt );
static CMonitor GetNearestMonitor( const CWnd* pWnd );

//is the specificed item visible on any monitor
static BOOL IsOnScreen( const POINT pt );
static BOOL IsOnScreen( const CWnd* pWnd );
static BOOL IsOnScreen( const LPRECT lprc );

//returns the rectangle encompassing all monitors
static void GetVirtualDesktopRect( LPRECT lprc );

//determines whether the given handle is a valid monitor handle
static BOOL IsMonitor( const HMONITOR hMonitor );
static CMonitor GetPrimaryMonitor();
static BOOL AllMonitorsShareDisplayFormat();

static int GetMonitorCount();

CMonitor is a wrapper around an HMONITOR handle (returned from EnumDisplayMonitors) and the GetMonitorInfo function. With CMonitor you can get at the characteristics of a given monitor.

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//The interface of CMonitor            
void Attach( const HMONITOR hMonitor );
HMONITOR Detach();

void ClipRectToMonitor( LPRECT lprc, 
                        const BOOL UseWorkAreaRect = FALSE ) const;
void CenterRectToMonitor( LPRECT lprc, 
                          const BOOL UseWorkAreaRect = FALSE ) const;
void CenterWindowToMonitor( CWnd* const pWnd,
                            const BOOL UseWorkAreaRect = FALSE ) const;

//creates a device context for the monitor - the client is responsible for 
// DeleteDC
HDC CreateDC() const;

void GetMonitorRect( LPRECT lprc ) const;
//the work area is the monitor rect minus the start bar
void GetWorkAreaRect( LPRECT lprc ) const;

void GetName( CString& string ) const;

int GetBitsPerPixel() const;

//determines if the specified item on the monitor
BOOL IsOnMonitor( const POINT pt ) const;
BOOL IsOnMonitor( const CWnd* pWnd ) const;
BOOL IsOnMonitor( const LPRECT lprc ) const;

BOOL IsPrimaryMonitor() const;
BOOL IsMonitor() const;

CMonitorDC is a CDC derived class that represents a monitor specific device context. I haven't really gone to far with this class but it seemed like a logical part of the library.  

Known Limitations

CMonitor and CMonitors rely on the assumption that a monitor handle does not change. This has proved to be a safe assumption empirically but isn't nessecarily a guarantee.



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