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Posted 13 Sep 2006


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How to drag a virtual file from your app into Windows Explorer

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13 Sep 2006CPOL3 min read
Shows how to use CFSTR_FILEDESCRIPTOR and CFSTR_FILECONTENTS to perform drag/drop with virtual files

Image 1


I was working on an FTP client application where I needed to support drag/drop with Windows Explorer. Dropping files from Explorer into the client was trivial and didn't cause me too much trouble, but dropping files back into Explorer was not as easy. Using CF_HDROP was ruled out because the source file would not physically exist, as it would have to be downloaded from the FTP server before Explorer can get at it. After playing around with various stuff, I finally hit upon using CFSTR_FILEDESCRIPTOR and

, which suited my purposes. In this article, I'll demonstrate a simple dialog app that will let you drag non-existent files from your app into Explorer.

Basic Technique

  • Derive a class from COleDataSource
  • Allocate global memory and create data in CFSTR_FILEDESCRIPTOR format, and then use CacheGlobalData on the
    derived class
  • Override OnRenderFileData in this derived class
  • In the OnRenderFileData override, handle
    , and write directly to the CFSTR_FILECONTENTS

Creating the demo app

Generate a default Dialog based MFC application with Visual Studio 2005 (or an earlier version if you don't have 2005). Use the resource editor to add a List control to the dialog, and associate a DDX control variable with it of type CListCtrl and name it m_fileList. Now add the following code to the OnInitDialog to setup the List control.

BOOL CExplorerDelayDropDlg::OnInitDialog()

    . . .       


    m_fileList.InsertColumn(0, _T("File"), LVCFMT_LEFT,75);
    m_fileList.InsertColumn(1, _T("Details"), LVCFMT_LEFT,175);

    for(TCHAR c = _T('A'); c < _T('F'); c++)
        CString text1, text2;
        text2.Format(_T("File full of %cs"),c);
            m_fileList.InsertItem(c - _T('A'),text1),1,text2);

    return TRUE;  

The code merely fills up the List control with some dummy file names. Note how I have made a call to AfxOleInit (you don't need to do this if your app already supports OLE).

Deriving a class from COleDataSource

Since we are using delayed data rendering, we need to derive a class from COleDataSource so that we can override OnRenderFileData (the default version merely returns FALSE). So, the first step is to add a class named CMyOleDataSource (derived from

) to the project.

class CMyOleDataSource : public COleDataSource

Now, we need to add an override for OnRenderFileData as shown below.

BOOL CMyOleDataSource::OnRenderFileData(
    LPFORMATETC lpFormatEtc,CFile* pFile)
    // We only need to handle CFSTR_FILECONTENTS
    if(lpFormatEtc->cfFormat == 
        HGLOBAL hGlob = NULL;
        const int buffSize = 512;
        hGlob = GlobalAlloc(GMEM_FIXED, buffSize);
            LPBYTE pBytes = (LPBYTE)GlobalLock(hGlob);          
                // lpFormatEtc->lindex can be used to identify
                // the file that's being copied
                memset(pBytes, (int) m_Files.GetAt(
                    lpFormatEtc->lindex)[0], buffSize);
        // Need to return TRUE to indicate success to Explorer
        return TRUE;
    return COleDataSource::OnRenderFileData(
        lpFormatEtc, pFile);

Note how in the above code, I create dummy files by filling up 512 bytes with a specific character that identifies the file. In a more real life scenario, you'd have to retrieve a specific file, identified by the

parameter, and then retrieve that file from a remote source or perhaps from an archive.

Now, we just need to handle the LVN_BEGINDRAG notification as shown below. You can either use the Properties box to add a handler, or manually add an ON_NOTIFY handler in the dialog class.

void CExplorerDelayDropDlg::OnBeginDrag(NMHDR *pNMHDR, LRESULT *pResult)
    UINT uFileCount = m_fileList.GetSelectedCount();    

    // The CFSTR_FILEDESCRIPTOR format expects a 
    // FILEGROUPDESCRIPTOR structure followed by an
    // array of FILEDESCRIPTOR structures, one for
    // each file being dropped
    UINT uBuffSize = sizeof(FILEGROUPDESCRIPTOR) + 
        (uFileCount-1) * sizeof(FILEDESCRIPTOR);
    HGLOBAL hFileDescriptor = GlobalAlloc ( 
        GHND | GMEM_SHARE, uBuffSize );        

        FILEGROUPDESCRIPTOR* pGroupDescriptor = 
            (FILEGROUPDESCRIPTOR*) GlobalLock ( hFileDescriptor );
            // Need a pointer to the FILEDESCRIPTOR array
            FILEDESCRIPTOR* pFileDescriptorArray = 
                (FILEDESCRIPTOR*)((LPBYTE)pGroupDescriptor + sizeof(UINT));
            pGroupDescriptor->cItems = uFileCount;            

            POSITION pos = m_fileList.GetFirstSelectedItemPosition();
            int index = 0;
            while( NULL != pos )
                int nSelItem = m_fileList.GetNextSelectedItem( pos );
                lstrcpy ( pFileDescriptorArray[index].cFileName, 
                    m_fileList.GetItemText( nSelItem, 0 ) );
                pFileDescriptorArray[index].dwFlags = 
                pFileDescriptorArray[index].nFileSizeLow = 512;
                pFileDescriptorArray[index].nFileSizeHigh = 0;
                pFileDescriptorArray[index].dwFileAttributes = 
            GlobalFree ( hFileDescriptor );
    GlobalUnlock ( hFileDescriptor );       

    // For the CFSTR_FILEDESCRIPTOR format, we use
    // CacheGlobalData since we make that data available 
    // immediately
    FORMATETC etcDescriptor = { 
    m_DataSrc.CacheGlobalData ( RegisterClipboardFormat(
        CFSTR_FILEDESCRIPTOR), hFileDescriptor, &etcDescriptor );

    // For CFSTR_FILECONTENTS, we use DelayRenderFileData
    // as this data will have to come from a non-physical
    // device, like an FTP site, an add-on device, or an archive
    FORMATETC etcContents = { 
        CFSTR_FILECONTENTS), &etcContents);

    DROPEFFECT dwEffect = m_DataSrc.DoDragDrop ( 

    // Free memory in case of failure
    if(dwEffect == DROPEFFECT_NONE )
        GlobalFree( hFileDescriptor );
    *pResult = 0;


That's it. Obviously, this just shows the bare techniques. You'd need to write extra code to make the whole process smooth. For example, if you are pulling the file from a remote device, there'd be a delay before the file gets written, in which case, you'd need to show a progress bar, or ensure that your main app does not freeze up entirely. But the basic technique will remain the same.


For more on Drag/Drop with Explorer, read Mike Dunn's excellent article : How to Implement Drag and Drop Between Your Program and Explorer which explains how to use CF_HDROP to transfer existing files to Explorer.


  • Sep/13/2006 - Article first published


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Nish Nishant
United States United States
Nish Nishant is a Principal Software Architect based out of Columbus, Ohio. He has over 17 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish was a Microsoft Visual C++ MVP between 2002 and 2015.

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored C++/CLI in Action for Manning Publications in 2005, and had previously co-authored Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on and another 250+ blog articles on his WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : If you are interested in hiring Nish as a consultant, you can reach him via his google email id voidnish.

Company Website :

Comments and Discussions

Questionhi!How get to the destination address? Pin
forcj92817-Nov-13 14:53
Memberforcj92817-Nov-13 14:53 
Questionhow can I implement this in .Net or C# Pin
Edgar10527-May-12 8:39
MemberEdgar10527-May-12 8:39 
QuestionConnecting to a real file or delay loading a CF_HDROP Pin
Rick Pingry12-Nov-10 7:50
MemberRick Pingry12-Nov-10 7:50 
AnswerRe: Connecting to a real file or delay loading a CF_HDROP Pin
Rick Pingry17-Nov-10 6:57
MemberRick Pingry17-Nov-10 6:57 
GeneralRe: Connecting to a real file or delay loading a CF_HDROP Pin
Nish Nishant17-Nov-10 7:18
sitebuilderNish Nishant17-Nov-10 7:18 
GeneralMy vote of 5 Pin
Rick Pingry2-Nov-10 9:35
MemberRick Pingry2-Nov-10 9:35 
QuestionWhat can I do if I want to move a folder? Pin
Yu Tist12-May-09 3:26
MemberYu Tist12-May-09 3:26 
AnswerRe: What can I do if I want to move a folder? [modified] Pin
jerry_wangjh4-Dec-10 15:48
Memberjerry_wangjh4-Dec-10 15:48 
GeneralRe: What can I do if I want to move a folder? [modified] Pin
Lester_200811-Aug-12 23:16
MemberLester_200811-Aug-12 23:16 
AnswerRe: What can I do if I want to move a folder? Pin
Lester_200810-Aug-12 23:31
MemberLester_200810-Aug-12 23:31 
QuestionHow would I do this without MFC? Pin
NTDLS4-Mar-08 8:09
MemberNTDLS4-Mar-08 8:09 
GeneralRe: How would I do this without MFC? Pin
Michael Dunn18-Mar-08 8:49
sitebuilderMichael Dunn18-Mar-08 8:49 
QuestionHow do I drag a 4GB file? Pin
cmumford9-Aug-07 11:05
Membercmumford9-Aug-07 11:05 
AnswerRe: How do I drag a 4GB file? PinPopular
rfalck21-Aug-07 15:40
Memberrfalck21-Aug-07 15:40 
GeneralRe: How do I drag a 4GB file? Pin
rfalck22-Aug-07 16:41
Memberrfalck22-Aug-07 16:41 
GeneralRe: How do I drag a 4GB file? Pin
drichard283017-Sep-07 12:35
Memberdrichard283017-Sep-07 12:35 
GeneralRe: How do I drag a 4GB file? Pin
rfalck17-Sep-07 13:54
Memberrfalck17-Sep-07 13:54 
GeneralRe: How do I drag a 4GB file? Pin
cmumford25-Sep-07 3:51
Membercmumford25-Sep-07 3:51 
GeneralRe: How do I drag a 4GB file? Pin
Brian Pence27-Sep-07 9:57
MemberBrian Pence27-Sep-07 9:57 
GeneralRe: How do I drag a 4GB file? Pin
cmumford27-Sep-07 12:24
Membercmumford27-Sep-07 12:24 
GeneralRe: How do I drag a 4GB file? Pin
fabioglauser9-Oct-07 13:34
Memberfabioglauser9-Oct-07 13:34 
GeneralRe: How do I drag a 4GB file? Pin
drichard28307-Nov-07 6:17
Memberdrichard28307-Nov-07 6:17 
QuestionBuild Fails if "Use of MFC" changed to "Static Library" from "Shared DLL" Pin
Raptorikus19-Jul-07 7:15
MemberRaptorikus19-Jul-07 7:15 
AnswerRe: Build Fails if "Use of MFC" changed to "Static Library" from "Shared DLL" Pin
cmumford8-Aug-07 10:12
Membercmumford8-Aug-07 10:12 
QuestionWhere was the file dropped? Pin
rfalck1-May-07 15:04
Memberrfalck1-May-07 15:04 

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