PRB: Drag and Drop with TYMED_FILE Runs Out of File Handles

• Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52, 1.52b
• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0

SYMPTOMS

Calling Open() on the CFile pointer returned from COleDataObject::GetFileData() may return EMFILE. Usually, this will happen after repeated drag and drop operations and subsequent calls to GetFileData(). It is also possible to see memory leaks upon termination of the application, specifically, of memory blocks the size of a CFile object.

CAUSE

The documentation for COleDataObject::GetFileData() does not mention that the returned CFile pointer is owned by the caller. Therefore, it is the responsibility of the caller to call Close() on or simply delete that pointer. The reason for the EMFILE return code is that the files opened in the drag and drop operation are never closed and therefore the system handles are exhausted. Memory leaks are caused by not deleting the CFile objects that were dynamically allocated by GetFileData().

The documentation for COleDataObject::GetFileData provided with Visual C++ 4.0 has been improved. It mentions the fact that it is the responsibility of the caller to delete the returned CFile object, thereby closing the file.

RESOLUTION

Remember to delete the CFile pointer returned from GetFileData() after using it.

Sample Code

The following OnDrop() function definition illustrates the correct cleanup after using the CFile pointer returned from GetFileData().

   BOOL CMyView::OnDrop(COleDataObject* pDataObject, DROPEFFECT de,
                        CPoint point)
   {
       CFile * pFile = pDataObject->GetFileData(CF_HGLOBAL);
       char lpBuf[100];

       if (NULL != pFile)
           pFile->Read(lpBuf, 100);
       .
       .
       .
       // Make sure to delete the CFile *
       delete pFile;
       return TRUE;
   }