Using WM_COMMNOTIFY in an MFC Application

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows, version 3.1

SUMMARY

The Microsoft Foundation Classes (MFC) version 2.0 for Microsoft Windows version 3.1 does not encapsulate the WM_COMMNOTIFY message and the Windows Communications API. Therefore, MFC applications that need to use the Windows Communications API have to wrap that API into their classes. This article discusses the steps involved in that process.

MORE INFORMATION

The steps to incorporate WM_COMMNOTIFY-based communications support to your MFC-based Windows-based application are as follows:

1. Specify a handler for the WM_COMMNOTIFY message using the OnMessage macro. The code to implement this step is shown below:

         // In the class header file,
         // generated message map functions protected:
         //{{AFX_MSG(CMfccommView)
              afx_msg LRESULT OnCommNotify(WPARAM wParam, LPARAM lParam);
              <other handlers>
         //}}AFX_MSG
         DECLARE_MESSAGE_MAP()
   
         // In the source file defining the class:
         BEGIN_MESSAGE_MAP(CMfccommView, CView)
              //{{AFX_MSG_MAP(CMfccommView)
              ON_MESSAGE(WM_COMMNOTIFY, OnCommNotify)
              <other mappings>
              //}}AFX_MSG_MAP
         END_MESSAGE_MAP()
   
   

2. Write the handler procedure OnCommNotify() to trap the CN_EVENT, CN_RECEIVE, and CN_TRANSMIT notifications.

3. Because WM_COMMNOTIFY is a message, it is always sent to a window. If you do not already have a window handle to receive this message, use the Create() member of the CWnd class to create a hidden window (do not specify WS_VISIBLE) and use that handle (m_hWnd member) in the call to EnableCommNotification().

   NOTE: Remember that merely creating an instance of CWnd does not create a window. A common error is to derive a class from CWnd and to call EnableCommNotification() in the constructor of the derived class. This fails because the window handle, m_hWnd, is NULL at this point.

4. Because you cannot return values from the constructor, the constructor is not a suitable place for initializing the communications port and device. Perform all initializations outside of the constructor in a member function created for initialization purposes. Remember to check all the return values from the communications API.