Does anyone know which relation may exist between registration-free COM and drag/drop functionality?

Specifically, we have a huge C++ CAD/CAM application comprising a number of EXEs and several hundreds DLLs. Many of them serve as COM servers (both in-proc and out-of-proc) and/or clients, and also implement ActiveX controls.

The most of ActiveX controls and the main CMDIFrameWnd-based window of one of EXEs implement drag/drop functionality. ActiveX controls implement the both drop source and drop target, and the main window is only drop target, in particular, for files from Windows Explorer.

The drag/drop implementation is pretty standard and based on two data members derived from COleDataSource and COleDropTarget for drop source and drop target respectively. The COleDropTarget-derived member is registered with respective window in the window's OnCreate method. It also overrides OnDragEnter, OnDragOver and OnDrop methods in a similar way. Namely, the system-supplied COleDataObject parameter is asked for specific format (in particular, CF_HDROP), and in the case of positive answer, the data (e.g., file path) is extracted from the clipboard. The code looks like the following:

static FORMATETC g_FileFmt = {CF_HDROP, 0, DVASPECT_CONTENT, 0, TYMED_HGLOBAL};
    ....
// Inside OnDragEnter, OnDragOver or OnDrop method
STGMEDIUM stgmedium = {0,0,0};
if (pDataObject->IsDataAvailable(g_FileFmt.cfFormat))
{
    HRESULT hr = pDataObject->GetData(g_FileFmt.cfFormat, &stgmedium);
    HDROP hdrop = (HDROP)GlobalLock(stgmedium.hGlobal);
    if (hdrop != 0)
    {
        int FilesCount = DragQueryFile(hdrop, (UINT)-1, 0, 0);
        if (FilesCount != 0)
        {
            TCHAR FileName[_MAX_PATH];
            DragQueryFile(hdrop, 0, FileName, _MAX_PATH);
            // Check file extension and store the file name for farther use.
        }
        GlobalUnlock(hdrop);
    }
}

The drop source implementation is also straightforward and looks like the following:

void CDmDocListCtrl::OnBeginDrag(NMHDR* pNMHDR, LRESULT* pResult)
{
    NM_LISTVIEW* pNMListView = (NM_LISTVIEW*)pNMHDR;
    if (pNMListView->iItem != -1 && m_pOleDataSource && prv_BeginDrag())
    {
        DROPEFFECT DE = m_pOleDataSource->DoDragDrop(
            DROPEFFECT_COPY | DROPEFFECT_MOVE | DROPEFFECT_LINK, 0);
    }
    *pResult = 0;
}

where prv_BeginDrag() function collects dragged data, packs it and puts on the clipboard by calling SetData method from the m_pOleDataSource object's IDataObject interface.

The all this stuff worked perfectly until it was decided to make the whole application registration-free. It took me three months to force the application run isolated (without registration of COM components) by embedding manifests, launching out-of-proc COM servers on demand and altering CLSID of some classes in order to separate instances of the same server launched from different folders. At last it begins to work - but without drag/drop functionality, despite it wasn't even touched by my changes.

On the drop target side, when I drag file from Windows Explorer, depicted above call to COleDataObject::IsDataAvailable returns false, although before my changes returned true. At the same time, if I add a single line of code "DragAcceptFiles();" to the main window's OnCreate method, drag/drop begins working via the standard CFrameWnd's WM_DROPFILE message handler.

On the drop source side, the dragged data are successfully packed and placed on the clipboard, but COleDataSource::DoDragDrop method fails, because a call to ::DoDragDrop API inside MFC implementation returns REGDB_E_CLASSNOTREG "Class not registered" result.

It means, that COM activation changes somehow influence drag/drop behavior. How?

P.S. 1) The EXE, to which I drag files from Windows Explorer, has in its project properties "UAC Execution Level = asInvoker". As far as I understand, it tells that the EXE will run at the same UAC level as Windows Explorer when launched by double-click on the file.

2) Quite surprisingly, although drag/drop stopped working with symptoms described above, Copy/Paste continues work well, despite the both technologies have similar implementation.

3) I believe, that if find out when ::DoDragDrop API returns "Class not registered" error, and which class it is looking for, it would be possible to solve the problem.

Thanks for help, Ilia.

Solution

Following to MartinBa advice, I solved the problem with the help of Process Monitor. The Process Monitor showed me that while I drag an item in the ActiveX control (mentioned in the question), the system unsuccessfully tries get access to a class ID in the Registry. Looking for that ID, I found that it is really not class ID, but IDataObject interface ID. It was referenced in one of my manifest files.

The most of manifests I have written by hand, but a few, especially at the beginning of the project having no experience in the area, I generated automatically by Visual Studio from existing type library. In one of them Studio included the comInterfaceExternalProxyStub statement for a couple of system interfaces, in which proxyStubClsid32 element was (erroneously) equal to the interface ID.

I'm still not sure whether those system interfaces should present in the manifest; for example, the IDataObject is only mentioned as a method's parameter in one of IDL definitions. Anyway, I corrected only the proxyStubClsid32 value, and the problem disappeared...

The moral of this very painful for me story is to always check output of automatic tools...