DOCUMENT:Q119765 11-FEB-2002 [visualc] TITLE :INFO: Example DDX Routine for Access Memo Fields PRODUCT :Microsoft C Compiler PROD/VER::1.5,1.51,1.52,2.0,2.1,4.0,4.1,4.2,5.0,6.0 OPER/SYS: KEYWORDS:kbDatabase kbMFC kbODBC kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC410 kbVC420 ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - The Microsoft Foundation Classes (MFC), used with: - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.51, 1.52 - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0, 4.1 - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 ------------------------------------------------------------------------------- SUMMARY ======= An application, using the ODBC database classes of the Microsoft Foundation Classes, can create a CRecordset derived class object which associates a CString or a CLongBinary member variable with a memo field within an Access database. Technote #45, "MFC/Database Support for Long Varchar/Varbinary," available in the MFC Tech Notes helpfile in your Visual C++ program group discusses this. There are some limitations to using a CString object associated with a memo field, as opposed to a CLongBinary, though: 1. In the 16-bit versions of the Microsoft Foundation Classes, a CString object can only hold up to 32K of data, while a CLongBinary object can hold much more than 64K if needed. 2. The initial size allocation of the CString buffer is determined by the size of the memo field. Because the memo field isn't a fixed length, the length may be zero for a new record or may be very small in some cases. This can cause problems when using AddNew()/Edit() and Update(). As the user assigns data to the CString, the size of the CString buffer may change and the buffer for the CString will be re-allocated causing an assertion. 3. CStrings terminate with NULL characters and it is possible that memo fields may contain any binary data including NULLs. There is, however, no explicit DDX routine to read the data from a CLongBinary in a recordset and copy it to an edit control, or to read the contents of the edit control back into the CLongBinary. (Or a similar control, as 16-bit edit controls are limited to 64K.) This article gives one example of such a DDX routine which reads data from a CLongBinary into an edit control and back. NOTE: A bug exists when using the 16-bit Access ODBC driver which is included with Visual C++ 1.52 which does not allow mapping of CLongBinary objects to Memo fields. For additional information, please see the following article in the Microsoft Knowledge Base: Q130438 BUG: Program Crashes When Memo Field Mapped to ClongBinary MORE INFORMATION ================ NOTE: The sample code below must explicitly call SetFieldNull() and SetFieldDirty() to set the field as NOT Null and as dirty when reading data from the control. Without both of these calls, the field will not be updated in the database. The DDX routine shown, will need to be explicitly called in your CFormView or CRecordView derived class's DoDataExchange() member function outside of the AFX_DATA_MAP section. For example the following code associates a CRecordset member variable CLongBinary m_lbMemo with an edit control with the ID IDC_EDIT_MEMO: void CSampView::DoDataExchange(CDataExchange* pDX) { CRecordView::DoDataExchange(pDX); //{{AFX_DATA_MAP(CSampView) DDX_FieldText(pDX, IDC_EDIT_TITLE, m_pSet->m_strTitle, m_pSet); DDX_FieldText(pDX, IDC_EDIT_AREA, m_pSet->m_strArea, m_pSet); //}}AFX_DATA_MAP DDX_FieldMemo( pDX, IDC_EDIT_MEMO, m_pSet->m_lbMemo, m_pSet ); } REFERENCES ========== MFC Tech Notes (helpfile icon in your Visual C++ program group) - Tech note #26, "DDX and DDV Routines" has additional information on working with DDX routines. - Tech note #45, "MFC/Database Support for Long Varchar/Varbinary" has additional information on working with large data, CLongBinary, CByteArray, and CString. Sample Code ----------- /* Compile options needed: Default MFC application project options */ void DDX_FieldMemo( CDataExchange * pDX, int nIDEdit, CLongBinary & lbMemo, CRecordset * pRecordset ) { ASSERT_VALID( pRecordset ); UINT nLen; HGLOBAL hGlob; LPSTR lpStr; char * pStrWithNull; HWND hWndCtrl = pDX->PrepareEditCtrl(nIDEdit); if ( !hWndCtrl ) { ASSERT(FALSE); return; } if (pDX->m_bSaveAndValidate) { nLen = ::GetWindowTextLength(hWndCtrl); if ( nLen ) { hGlob = GlobalAlloc( GPTR | GMEM_SHARE, (DWORD)nLen ); if ( !hGlob ) { AfxThrowMemoryException(); } lpStr = (LPSTR)GlobalLock( hGlob ); if ( !lpStr ) { GlobalFree( hGlob ); AfxThrowMemoryException(); } // Allocate space for the '\0' string terminator // Throws exception if needed pStrWithNull = new char[nLen+1]; ::GetWindowText(hWndCtrl, pStrWithNull, nLen+1); // Cut off the null #ifndef _WIN32 _fmemcpy( lpStr, pStrWithNull, nLen ); #else memcpy( lpStr, pStrWithNull, nLen ); #endif delete [] pStrWithNull; GlobalUnlock( hGlob ); // Don't leave it locked. } else // Empty { nLen = 0; hGlob = NULL; } // Free memory we are replacing GlobalUnlock( lbMemo.m_hData ); GlobalFree( lbMemo.m_hData ); // Put in new data lbMemo.m_dwDataLength = (DWORD) nLen; lbMemo.m_hData = hGlob; if (nLen == 0) { if ( pRecordset->IsFieldNullable(&lbMemo) ) pRecordset->SetFieldNull( &lbMemo, TRUE); } else { // It is required that we explicitly set it Dirty // and NOT Null pRecordset->SetFieldNull( &lbMemo, FALSE ); pRecordset->SetFieldDirty( &lbMemo, TRUE ); } } else // Reading data from recordset into control { nLen = (UINT)lbMemo.m_dwDataLength; if ( nLen ) { lpStr = (LPSTR)GlobalLock( lbMemo.m_hData ); // Throws exception if needed pStrWithNull = new char[nLen+1]; #ifndef _WIN32 _fmemcpy( pStrWithNull, lpStr, nLen ); #else memcpy( pStrWithNull, lpStr, nLen ); #endif pStrWithNull[nLen] = 0; // Set '\0' at end of string SetWindowText( hWndCtrl, pStrWithNull ); delete [] pStrWithNull; GlobalUnlock( lbMemo.m_hData ); } else { SetWindowText( hWndCtrl, "" ); } } } Additional query words: SQL_LONGVARCHAR SQL_LONGVARBINARY MfcDatabase kbMFC kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbVC600 ====================================================================== Keywords : kbDatabase kbMFC kbODBC kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbGrpDSODBC Technology : kbAudDeveloper kbMFC Version : :1.5,1.51,1.52,2.0,2.1,4.0,4.1,4.2,5.0,6.0 Issue type : kbinfo ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. 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