Using a Message-Only Window to Receive Device Change Notification Messages [modified]

Jim Fell

I'm trying to get write a message-only window to receive device change notification messages for a USB device. I am using MFC, C++, and Visual Studio 2008. Everything compiles, and it runs without crashing or locking up, but the event handler is never triggered. The device of interest is installed on Windows as a virtual COM port. My main application instantiates the class described below then waits for a character input from the keyboard polling using a while loop. It is during this wait time that I remove and insert my USB device expecting the event to get fired.

class CMessageOnlyWindow : public CWnd
{
DECLARE_DYNAMIC(CMessageOnlyWindow)
private:
DEV_BROADCAST_DEVICEINTERFACE * _pDevIF; // The notification filter.
HDEVNOTIFY _hNotifyDev; // The device notification handle.
public:
CMessageOnlyWindow();
virtual ~CMessageOnlyWindow();
protected:
afx_msg BOOL OnDeviceChange( UINT nEventType, DWORD dwData );
private:
void RegisterNotification( void );
void UnregisterNotification( void );
protected:
DECLARE_MESSAGE_MAP() // Must be last.
};

For simplicity, I've removed all the cleanup and error-handling:

DEFINE_GUID(GUID_INTERFACE_CP210x, 0x993f7832, 0x6e2d, 0x4a0f, \
0xb2, 0x72, 0xe2, 0xc7, 0x8e, 0x74, 0xf9, 0x3e);

IMPLEMENT_DYNAMIC(CMessageOnlyWindow, CWnd)

CMessageOnlyWindow::CMessageOnlyWindow()
{
CString cstrWndClassName = ::AfxRegisterWndClass( NULL );
BOOL bCreated = this->CreateEx( 0, cstrWndClassName,
L"CMessageOnlyWindow", 0, 0, 0, 0, 0, HWND_MESSAGE, 0 );
this->RegisterNotification();
}

CMessageOnlyWindow::~CMessageOnlyWindow() {}

BEGIN_MESSAGE_MAP(CMessageOnlyWindow, CWnd)
ON_WM_DEVICECHANGE()
END_MESSAGE_MAP()

afx_msg BOOL CMessageOnlyWindow::OnDeviceChange( UINT nEventType, DWORD dwData )
{
switch ( nEventType ) // <-- Never gets here.
{
case DBT_DEVICEARRIVAL:
break;

case DBT\_DEVICEREMOVECOMPLETE:
    break;

default:
    break;
}

return TRUE;

}

void CMessageOnlyWindow::RegisterNotification(void)
{
_pDevIF = (DEV_BROADCAST_DEVICEINTERFACE *)malloc( sizeof(DEV_BROADCAST_DEVICEINTERFACE) );
memset( _pDevIF, 0, sizeof(DEV_BROADCAST_DEVICEINTERFACE) );
_pDevIF->dbcc_size = sizeof(DEV_BROADCAST_DEVICEINTERFACE);
_pDevIF->dbcc_devicetype = DBT_DEVTYP_DEVICEINTERFACE;
_pDevIF->dbcc_classguid = GUID_INTERFACE_CP210x;
_hNotifyDev = Regi

Luc Pattyn

AFAIK the window will only get messages when it is "visible" and non-minimized, which means you should show it on the desktop, at any location you like, including (-1000,-1000) to keep it out of sight. :)

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Jim Fell

Thanks, Luc. I tried specifying a location with the same lackluster results:

// BOOL bCreated = this->CreateEx( 0, cstrWndClassName,
// L"CMessageOnlyWindow", 0, 0, 0, 0, 0, HWND_MESSAGE, 0 );
BOOL bCreated = this->CreateEx( 0, cstrWndClassName,
L"CMessageOnlyWindow", -1000, -1000, 100, 100, 0, HWND_MESSAGE, 0 );

Luc Pattyn

I was thinking you had to actually show the window (hence the off-screen coordinates), howver my source code (which is C#) doesn't, and works well. I'm afraid I can't help you. :)

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Luc Pattyn

Hi again, I happened to require detection of memory stick insertions (in C# again), and experimented a bit more. My conclusion is the app needs a window that is shown, non-minimized, possibly off-screen, and then overriding the wndProc yields the necessary events. So I chose "manual window position" with a bounding rectangle of (-32000,-32000,100,100) and it works well; it does not when minimized or not shown at all. :)

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Please use <PRE> tags for code snippets, they preserve indentation, and improve readability.