PRB: Calling CDatabase::Open in a USRDLL Causes CDBException

• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
  • Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1

SYMPTOMS

Calling the CDatabase::Open function in a USRDLL and using a parameter list of (NULL, FALSE, FALSE, "ODBC;") may generate a CDBException with it's m_strError member set to a value of "Dialog failed."

CAUSE

When the CDatabase::Open function is called with a value of NULL for the first parameter and the default values for the remaining parameters, an SQL Data Source dialog is displayed to the user allowing them to choose the data source they want to open.

When this is done in the context of a USRDLL, the CDBException is generated because CDatabase::Open() attempts to specify the main or active window of the CWinApp or CWinThread object as the parent of the SQL Data Source dialog. In a USRDLL, the main or active window of the CWinApp or CWinThread object is typically NULL.

RESOLUTION

Avoid the problem by using the main window of the calling application as the parent of the SQL Data Source dialog. The sample code listed below illustrates one way to implement this solution. The sample DLL exports one function, OpenDatabase(), which displays a dialog with a list of data sources. The OpenDatabase() function accepts a single parameter of type HWND, which is then used as the handle to the parent window of the SQL Data Source dialog.

STATUS

This behavior was by design, however it was modified in Visual C++ version 2.2. to eliminate the problem.

MORE INFORMATION

Sample Code

/* 16-bit Compile options needed: /GD
*  16-bit Link options needed:    /LIB:"odbc"
*/ 
#ifndef _WINDLL        // Make this work with WIN32 and WIN16
#define _WINDLL
#endif
#ifndef _USRDLL
#define _USRDLL
#endif
#include <afxwin.h>
#include <afxext.h>
#include <afxdb.h>
class MyDLL : public CWinApp
{
public:
    BOOL InitInstance();
};
#ifdef WIN32    // Make this work with WIN32 and WIN16
extern "C" __declspec(dllexport) int
#else
extern "C" int _export
#endif
FAR PASCAL OpenDatabase(HWND hWndParent /* Needs a parent window */)
{
    TRY
    {
        // Before calling MyDb.Open setup a parent window
        AfxGetApp()->m_pMainWnd = new CWnd;
        AfxGetMainWnd()->Attach(hWndParent);
        CDatabase MyDb;
        // Present list of ODBC sources
        MyDb.Open(NULL,FALSE,FALSE,"ODBC;");
        MyDb.Close();
        // Clean Up
        AfxGetMainWnd()->Detach();
        delete AfxGetMainWnd();
        AfxGetApp()->m_pMainWnd = NULL;
    }
    CATCH_ALL(e)
    {
        if ( AfxGetMainWnd()->GetSafeHwnd() != NULL )
        {
            // Clean Up
            AfxGetMainWnd()->Detach();
            delete AfxGetMainWnd();
            AfxGetApp()->m_pMainWnd = NULL;
        }
        // Exception caused a failure
        return FALSE;
    }
    END_CATCH_ALL
    return TRUE;
}
BOOL MyDLL::InitInstance()
{
    return TRUE;
}
MyDLL theDLL; 

Additional query words: 1.00 1.50 2.00 2.10 2.50 2.51 2.52 3.00 3.10

Keywords : kbcode
Version : 1.00 1.50 1.51 1.52 | 1.00 2.00
Platform : NT WINDOWS
Issue type :