PRB: MFC DAO Functions Accepting COleVariant Strings May Fail

ID: Q140599

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 4.2b, 5.0

SYMPTOMS

The MFC DAO Classes contain methods that accept COleVariant arguments. For example, CDaoRecordset::Seek(), CDaoRecordset::SetFieldValue(), and CDaoRecordset::SetBookMark() accept COleVariant arguments. You may find that these functions do not perform correctly if you are building a non-UNICODE build of your application and you are assigning a string to a COleVariant and passing the variant to one of these methods.

For example, look at the following code, which is shown in MFC Technote #53:


   COleVariant varFieldValue;
   rs.AddNew();
   varFieldValue = _T("MSFT");
   rs.SetFieldValue(_T("Customer_ID"), varFieldValue);
   varFieldValue = _T("Microsoft");
   rs.SetFieldValue(_T("Customer_Name"), varFieldValue);
   rs.Update();

This code causes only the first letters of each string to be copied to the database with a non-UNICODE build. In this case, the letter M will be copied to both fields.

CAUSE

MFC creates ANSI DAO objects when building non-UNICODE builds and creates UNICODE DAO objects when building UNICODE builds. For example, look at the following code in AfxDaoInit():


      GUID guidEngine;
#ifdef _UNICODE
      guidEngine = IID_IDAODBEngineW;
#else
      guidEngine = IID_IDAODBEngine;
#endif

The ANSI DAO objects require ANSI BSTRs for any strings that they receive. When you initialize a COleVariant object using:


   COleVariant varFieldValue(_T("some string"));

-or-


   COleVariant varFieldValue;
   varFieldValue = _T("MSFT");

The COleVariant will contain a UNICODE BSTR regardless of whether the build is UNICODE or non-UNICODE. This is not what DAO expects.

RESOLUTION

There are two techniques you can use to initialize the COleVariant properly. If you are only concerned about setting the BSTR value of the COleVariant at construction time, use this code:


    COleVariant varFieldValue(_T("SomeString"), VT_BSTRT);

The VT_BSTRT constant specifies that the BSTR will be a UNICODE BSTR with a UNICODE build and it will be a ANSI BSTR in a non-UNICODE build.

If you need to change the value of the variant often, you may want to create a helper function:


    void FillVariant(COleVariant & var, LPCTSTR lpszSrc)
    {
        var.vt=VT_BSTR;
        if (lpszSrc==NULL)
            var.bstrVal=NULL;
        else
        {
    #ifndef _UNICODE
        int nLen = lstrlen(lpszSrc);
        var.bstrVal = SysAllocStringByteLen(lpszSrc, nLen);
    #else
        var.bstrVal = SysAllocString(lpszSrc);
    #endif
        }
    }

With Visual C++ 4.2 or higher, COleVariant contains a SetString() function that can be used instead of the FillVariant() function above.

STATUS

This behavior is by design.

Additional query words: Seek SetFieldValue SetCacheStart SetBookMark SetParamValue

Keywords : kbprg kbDAO kbDatabase kbMFC kbVC
Version : 4.0 4.1 4.2 4.2b 5.0
Platform : NT WINDOWS
Issue type : kbprb