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.