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

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

Introduction

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 CFSTR_FILECONTENTS, 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 COleDataSource derived class
  • Override OnRenderFileData in this derived class
  • In the OnRenderFileData override, handle CFSTR_FILECONTENTS, 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()
{
    CDialog::OnInitDialog();

    . . .       

    AfxOleInit();

    m_fileList.SetExtendedStyle(LVS_EX_FULLROWSELECT);
    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;
        text1.Format(_T("%c.txt"),c);
        text2.Format(_T("File full of %cs"),c);
        m_fileList.SetItemText(
            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 COleDataSource) to the project.

class CMyOleDataSource : public COleDataSource
{
    DECLARE_DYNAMIC(CMyOleDataSource)

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 == 
        RegisterClipboardFormat(CFSTR_FILECONTENTS))
    {   
        HGLOBAL hGlob = NULL;
        const int buffSize = 512;
        hGlob = GlobalAlloc(GMEM_FIXED, buffSize);
        if(hGlob)
        {
            LPBYTE pBytes = (LPBYTE)GlobalLock(hGlob);          
            if(pBytes)
            {
                // lpFormatEtc->lindex can be used to identify
                // the file that's being copied
                memset(pBytes, (int) m_Files.GetAt(
                    lpFormatEtc->lindex)[0], buffSize);
                pFile->Write(pBytes,buffSize);              
            }
            GlobalUnlock(hGlob);
        }
        GlobalFree(hGlob);
        // 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 lindex 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 );        

    if(hFileDescriptor)
    {
        FILEGROUPDESCRIPTOR* pGroupDescriptor = 
            (FILEGROUPDESCRIPTOR*) GlobalLock ( hFileDescriptor );
        if(pGroupDescriptor)
        {
            // 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;
            m_DataSrc.m_Files.RemoveAll();
            while( NULL != pos )
            {   
                int nSelItem = m_fileList.GetNextSelectedItem( pos );
                ZeroMemory(&pFileDescriptorArray[index], 
                    sizeof(FILEDESCRIPTOR));
                lstrcpy ( pFileDescriptorArray[index].cFileName, 
                    m_fileList.GetItemText( nSelItem, 0 ) );
                m_DataSrc.m_Files.Add(
                    pFileDescriptorArray[index].cFileName);
                pFileDescriptorArray[index].dwFlags = 
                    FD_FILESIZE|FD_ATTRIBUTES;
                pFileDescriptorArray[index].nFileSizeLow = 512;
                pFileDescriptorArray[index].nFileSizeHigh = 0;
                pFileDescriptorArray[index].dwFileAttributes = 
                    FILE_ATTRIBUTE_NORMAL;
                index++;
            }
        }
        else
        {
            GlobalFree ( hFileDescriptor );
        }
    }
    GlobalUnlock ( hFileDescriptor );       

    // For the CFSTR_FILEDESCRIPTOR format, we use
    // CacheGlobalData since we make that data available 
    // immediately
    FORMATETC etcDescriptor = { 
        RegisterClipboardFormat(CFSTR_FILEDESCRIPTOR), 
        NULL, DVASPECT_CONTENT, -1, TYMED_HGLOBAL };
    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 = { 
        RegisterClipboardFormat(CFSTR_FILECONTENTS), 
        NULL, DVASPECT_CONTENT, -1, TYMED_HGLOBAL };
    m_DataSrc.DelayRenderFileData(RegisterClipboardFormat(
        CFSTR_FILECONTENTS), &etcContents);

    DROPEFFECT dwEffect = m_DataSrc.DoDragDrop ( 
        DROPEFFECT_COPY | DROPEFFECT_MOVE );

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

Conclusion

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.

Reference

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.

History

  • Sep/13/2006 - Article first published

License

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

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About the Author

Nish Sivakumar

United States United States
Nish is a real nice guy who has been writing code since 1990 when he first got his hands on an 8088 with 640 KB RAM. Originally from sunny Trivandrum in India, he has been living in various places over the past few years and often thinks it’s time he settled down somewhere.
 
Nish has been a Microsoft Visual C++ MVP since October, 2002 - awfully nice of Microsoft, he thinks. He maintains an MVP tips and tricks web site - www.voidnish.com where you can find a consolidated list of his articles, writings and ideas on VC++, MFC, .NET and C++/CLI. Oh, and you might want to check out his blog on C++/CLI, MFC, .NET and a lot of other stuff - blog.voidnish.com.
 
Nish loves reading Science Fiction, P G Wodehouse and Agatha Christie, and also fancies himself to be a decent writer of sorts. He has authored a romantic comedy Summer Love and Some more Cricket as well as a programming book – Extending MFC applications with the .NET Framework.
 
Nish's latest book C++/CLI in Action published by Manning Publications is now available for purchase. You can read more about the book on his blog.
 
Despite his wife's attempts to get him into cooking, his best effort so far has been a badly done omelette. Some day, he hopes to be a good cook, and to cook a tasty dinner for his wife.

Comments and Discussions

 
Questionhi!How get to the destination address? Pinmemberforcj92817-Nov-13 14:53 
Questionhow can I implement this in .Net or C# PinmemberEdgar10527-May-12 8:39 
QuestionConnecting to a real file or delay loading a CF_HDROP PinmemberRick Pingry12-Nov-10 7:50 
AnswerRe: Connecting to a real file or delay loading a CF_HDROP PinmemberRick Pingry17-Nov-10 6:57 
GeneralRe: Connecting to a real file or delay loading a CF_HDROP PinmvpNishant Sivakumar17-Nov-10 7:18 
GeneralMy vote of 5 PinmemberRick Pingry2-Nov-10 9:35 
QuestionWhat can I do if I want to move a folder? PinmemberYu Tist12-May-09 3:26 
AnswerRe: What can I do if I want to move a folder? [modified] Pinmemberjerry_wangjh4-Dec-10 15:48 
GeneralRe: What can I do if I want to move a folder? [modified] PinmemberLester_200811-Aug-12 23:16 
AnswerRe: What can I do if I want to move a folder? PinmemberLester_200810-Aug-12 23:31 
QuestionHow would I do this without MFC? PinmemberNTDLS4-Mar-08 8:09 
GeneralRe: How would I do this without MFC? PinmvpMichael Dunn18-Mar-08 8:49 
QuestionHow do I drag a 4GB file? Pinmembercmumford9-Aug-07 11:05 
AnswerRe: How do I drag a 4GB file? Pinmemberrfalck21-Aug-07 15:40 
GeneralRe: How do I drag a 4GB file? Pinmemberrfalck22-Aug-07 16:41 
GeneralRe: How do I drag a 4GB file? Pinmemberdrichard283017-Sep-07 12:35 
GeneralRe: How do I drag a 4GB file? Pinmemberrfalck17-Sep-07 13:54 
GeneralRe: How do I drag a 4GB file? Pinmembercmumford25-Sep-07 3:51 
GeneralRe: How do I drag a 4GB file? PinmemberBrian Pence27-Sep-07 9:57 
GeneralRe: How do I drag a 4GB file? Pinmembercmumford27-Sep-07 12:24 
GeneralRe: How do I drag a 4GB file? Pinmemberfabioglauser9-Oct-07 13:34 
GeneralRe: How do I drag a 4GB file? Pinmemberdrichard28307-Nov-07 6:17 
QuestionBuild Fails if "Use of MFC" changed to "Static Library" from "Shared DLL" PinmemberRaptorikus19-Jul-07 7:15 
AnswerRe: Build Fails if "Use of MFC" changed to "Static Library" from "Shared DLL" Pinmembercmumford8-Aug-07 10:12 
QuestionWhere was the file dropped? Pinmemberrfalck1-May-07 15:04 

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