Microsoft KB Archive/123652

= FIX: Update Fails to Write Changed Text Field =

Article ID: 123652

Article Last Modified on 11/21/2006

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APPLIES TO


 * Microsoft Foundation Class Library 4.2, when used with:
 * Microsoft Visual C++ 2.0 Professional Edition

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This article was previously published under Q123652



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); ... This code was removed in Visual C++ version 2.0. The code checks to see if a CString is empty and sets or resets the null flag accordingly. Without this code, the null flag for a field will remain set even though you may change the contents of a CString field variable. Because this code is no longer present in the Visual C++ version 2.0 database classes code, you will have to set or reset the null flag for a text field in your code.



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.

