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Using DAO from a DLL

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Microsoft's Visual C++ on-line help says that DAO is safe to use in DLL's. Therefore, I created a DLL which I use to access the same database my main application thread is accessing, in order to do a background report with one worker thread..

From the main application's point of view it is creating wrapper objects in a DLL. The wrapper objects, in turn, access the database and return the results to the application.

It is a single-threaded DLL but it could probably be modified to be thread-safe by using locks.

I list the code for one wrapper class here but you could put many different recordset wrappers into the DLL project.

Here's some of the code in my DLL:

class CMyRecordsetWrapper : public CCmdTarget
{
 DECLARE_DYNCREATE(CMyRecordsetWrapper)

 // protected constructor used by dynamic creation
 CMyRecordsetWrapper();           
 CMyRecordsetWrapper(CString filename);

// Attributes
public:
 CMyRecordset* myrecordset;
 long	m_index;
 long	m_time;
 long	m_eventtype; 
 CString	m_title;
 long	m_binarydataindex;
 long	m_offset;
 long	m_length;

// Operations
public:
 void Open(int thetype, LPCTSTR sqlstring, int themode);
 BOOL IsBOF();
 BOOL IsEOF();
 void MoveFirst();
 void MoveLast();
 LONG GetRecordCount();
 void Close();
 void MoveNext();
 BOOL FindNext(LPCTSTR lpszFilter);
 BOOL FindPrev(LPCTSTR lpszFilter);
 BOOL CanRestart();
 void copyem();
 void opentherecordset();

// Overrides
 // ClassWizard generated virtual function overrides
 //{{AFX_VIRTUAL(CMyRecordsetWrapper)
 //}}AFX_VIRTUAL

// Implementation
 virtual ~CMyRecordsetWrapper();

protected:
 // Generated message map functions
 //{{AFX_MSG(CMyRecordsetWrapper)
  // NOTE - the ClassWizard will add and remove member functions here.
 //}}AFX_MSG

 DECLARE_MESSAGE_MAP()
};

CMyRecordsetWrapper::CMyRecordsetWrapper(CString filename)
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  if (theApp.databaseopen == 0)
  {
    AfxDaoInit( );
    theApp.eventdatabase = new CDaoDatabase;
    theApp.eventdatabase->Open(filename);
  }
  // increment reference count
  theApp.databaseopen++;
  opentherecordset();
}

CMyRecordsetWrapper::~CMyRecordsetWrapper()
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  delete myrecordset;
  theApp.databaseopen--;
  if (theApp.databaseopen == 0)
  {
    theApp.eventdatabase->Close();
    delete theApp.eventdatabase;
    AfxDaoTerm();
  }
}

void CMyRecordsetWrapper::opentherecordset()
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  myrecordset = new CMyRecordset(theApp.eventdatabase);
}

void CMyRecordsetWrapper::Close()
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  myrecordset->Close();
}

void CMyRecordsetWrapper::MoveFirst()
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  myrecordset->MoveFirst();
  copyem();
}

void CMyRecordsetWrapper::copyem()
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  m_index = myrecordset->m_index;
  m_time = myrecordset->m_time;
  m_eventtype = myrecordset->m_eventtype;
  m_title = myrecordset->m_title;
  m_binarydataindex = myrecordset->m_binarydataindex;
  m_offset = myrecordset->m_offset;
  m_length = myrecordset->m_length;
}


This article was originally published on March 7th, 2000

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