PRB: MFC DAO Functions Accepting COleVariant Strings May Fail

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, 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();

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;

      guidEngine = IID_IDAODBEngineW;

      guidEngine = IID_IDAODBEngine;

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");

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);

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
        }
    }

STATUS

This behavior is by design.