Using CLongBinary for BLOBs

Download demo project - 29 Kb

At times, storing images and other types of documents like word document, or excel spread sheet or even a compressed zip file in the database is inevitable. MFC provides a supporting class to handle these BLOBs called CLongBinary. It is just a wrapper around the HGLOBAL of WIN32. But how do you use it? The wizard creates a member variable of type CLongBinary in your record set object and also creates the corresponding RFX routine (RFX_LongBinary).

Okay, does it mean that you can use this member just like any other variable? No, That's where this document comes in handy. For regular data types, all you have to is create an instance of the recordset object, assign the values and call Update. But for BLOBs, you have to do something special. For inserting BLOBs, you have to remember 3 points.

1. Allocate and lock the storage buffer using GlobalAlloc() and GlobalLock() calls
2. Prepare the recordset for blob operation using SetFieldDirty() and SetFieldNull() calls
3. Unlock the storage buffer

By default, the destructor of the CLongBinary will delete the buffer using GlobalFree() call. So you don't have to bother about freeing the memory. Reading from the database is pretty simple. All you have to do is lock the buffer.

Following code snippet shows how to do this in the actual code:

// Store the Image to database table
// 
try
{
    dbImages.Open();
    dbImages.AddNew();

    CFile	fileImage;
    CFileStatus	fileStatus;

    fileImage.Open("c:\\winnt\\winnt256.bmp", CFile::modeRead);
    fileImage.GetStatus(fileStatus);

    dbImages.m_BLOBName = fileImage.GetFileTitle();
    dbImages.m_BLOBImage.m_dwDataLength = fileStatus.m_size;

    HGLOBAL hGlobal		= GlobalAlloc(GPTR,fileStatus.m_size);
    dbImages.m_BLOBImage.m_hData = GlobalLock(hGlobal);

    fileImage.ReadHuge(dbImages.m_BLOBImage.m_hData,fileStatus.m_size);

    dbImages.SetFieldDirty(&dbImages.m_BLOBImage);
    dbImages.SetFieldNull(&dbImages.m_BLOBImage,FALSE);
    dbImages.Update();

    GlobalUnlock(hGlobal);

    dbImages.Close();

    pList->InsertItem(0,fileImage.GetFileTitle());
}
catch(CException* pE)
{
    pE->ReportError();
    pE->Delete();
    return;
}

 

// To restore image from db table
CdbImages   dbImages(&theApp.m_DB);
CString     strFileName = pList->GetItemText(nIndex,0);
dbImages.m_strFilter.Format("BLOBName = '%s'",strFileName);
try
{
    dbImages.Open();
    if  (dbImages.IsEOF())
        AfxMessageBox("Unable to get image from db");
    else
    {
        char    tmpPath[_MAX_PATH+1];
        GetTempPath(_MAX_PATH,tmpPath);

        strFileName.Insert(0,tmpPath);
        
        CFile	outFile(strFileName,CFile::modeCreate|CFile::modeWrite);
        LPSTR	buffer = (LPSTR)GlobalLock(dbImages.m_BLOBImage.m_hData);
        outFile.WriteHuge(buffer,dbImages.m_BLOBImage.m_dwDataLength);
        GlobalUnlock(dbImages.m_BLOBImage.m_hData);
        outFile.Close();

        theApp.OpenDocumentFile(strFileName);
    }

    dbImages.Close();

}
catch(CException* pE)
{
    pE->ReportError();
    pE->Delete();
    return;
}

The demo project uses an Access database to store the images. This project also demonstrates the following:

• DSN - less Connection to MS Access database
• Dialog Bars
• Image Display in CScrollView

The images are displayed using the simplest technique of using BitBlt() call. It does not use any Palettes because it is beyond the scope of this article.