INFO: Example DDX Routine for Access Memo Fields

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52 - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 4.0,

       4.1, 4.2, 5.0
  

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.

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:

   ARTICLE-ID: Q130438
   TITLE     : BUG: Program Crashes When Memo Field Mapped to ClongBinary

MORE INFORMATION

Note that 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, "" );
           }
       }
   }