FIX: Update() Fails to Write Changed Text Field

The information in this article applies to:

   The Microsoft Foundation Classes (MFC) included with:
     - Microsoft Visual C++, 32-bit Edition, version 2.0

SYMPTOMS

The CRecordset::Update() function fails to correctly update a text field if that field is changed from a null value to a non-null value. The Update() function may return FALSE or, if the dirty flag is set for the field, the Update() function may return TRUE but a null value will be written to the record instead of the expected non-null value.

For example, the following code worked with Visual C++ version 1.50 or 1.51, but it may fail in Visual C++ version 2.0 given the conditions described above:

   recordset.Edit();
   recordset.Field1String="SomeText";
   recordset.Update();

CAUSE

The RFX_Text() function, which maps a CString to a text field, has changed in Visual C++ version 2.0. In Visual C++ version 1.51, you'll see the following code in the MarkForUpdate operation of the RFX_Text() function:

  case CFieldExchange::MarkForUpdate:
        {
            if (value.IsEmpty())
                pFX->m_prs->SetFieldFlags(nField,
                    AFX_SQL_FIELD_FLAG_NULL, pFX->m_nFieldType);
            else
                pFX->m_prs->ClearFieldFlags(nField,
                    AFX_SQL_FIELD_FLAG_NULL, pFX->m_nFieldType);
            ...

RESOLUTION

If you use a CRecordView for retrieving data, you won't notice the bug because a CRecordView sets or resets a field's null flag depending on whether a user has entered text into the edit control representing that field. The DDX_FieldText() function does the work of setting the field's null flag.

For example, in the AfxFieldText() function, which gets called from the DDX_FieldText() functions, you'll see this code:

***** BOOL AFXAPI AfxFieldText(CDataExchange* pDX, int nIDC, void* pv,

    CRecordset* pRecordset)

    ASSERT_VALID(pRecordset);

    HWND hWndCtrl = pDX->PrepareEditCtrl(nIDC);
    char szT[2];
    if (pDX->m_bSaveAndValidate)
    {
        ::GetWindowText(hWndCtrl, szT, sizeof(szT));
        if (szT[0] == '\0')
        {
            if (pRecordset->IsFieldNullable(pv))
            {
                pRecordset->SetFieldNull(pv);
                return TRUE;
            }
        }
        else
            pRecordset->SetFieldNull(pv, FALSE);
    }
    else
    {
        if (!pRecordset->IsOpen() || pRecordset->IsFieldNull(pv))
        {
            szT[0] = '\0';
            AfxSetWindowText(hWndCtrl, szT);
            return TRUE;
        }
    }
    return FALSE;

You can see how the null flag for the field is set or reset. You can write similar code to handle setting or resetting the null flag in your code.

As an alternative, you can create a new RFX function, and insert the code shown above for the MarkForUpdate operation. Replace the call to RFX_Text() in your CRecordset's DoFieldExchange() function with your new RFX function.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 2.1.

REFERENCES

For more information about Record Field Exchange (RFX) functions see the following references:

    MFC Technote #43 - "RFX Routines"
    MFC Encyclopedia articles on RFX in the online books