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Using CLongBinary for BLOBs

, 30 Nov 1999
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  • Download demo project - 29 Kb
  • Sample Image - UsingBlob.gif

    <!-- Article Starts -->

    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.

    License

    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

    A list of licenses authors might use can be found here

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    About the Author

    Shekar Narayanan
    United States United States
    No Biography provided

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    Comments and Discussions

     
    GeneralReally Good Pin
    manish rastogi2-Jul-08 23:48
    membermanish rastogi2-Jul-08 23:48 

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