IDataObjectAsyncCapability not being handled by Outlook in OLE Drag and Drop - node.js

Question

I work on an electron application, and we would like to support dragging and dropping .msg files from our app into Outlook (or the Windows shell, or wherever, but primarily Outlook). The files will not necessarily be available before the user starts the drag and drop operation, and will need to be downloaded once the user begins the drag and drop. This means we can't use electron's inbuilt startDrag as it requires that the files are already in the filesystem, so I have implemented a node c++ addon to handle it using the OLE Drag and Drop API.
The data extraction (from my app to Outlook) needs to occur asynchronously, otherwise node's event loop will be blocked, halting the download. This means that the data extraction has to occur on another thread so that the event loop can continue actually downloading the data (currently doing the download in c++ is not an option, it has to occur in node). So I have implemented IDataObjectAsyncCapability on my DataObject, to indicate that I support asynchronous data extraction (it would be even nicer to be able to indicate that I don't support synchronous), but Outlook won't even query for the Interface, let alone do the data extraction asynchronously. The Windows shell does however query for the interface, but after the call to GetData, it still performs the extraction synchronously inside DoDragDrop, without even invoking any of the IDataObjectAsyncCapability methods. Is there something lacking in my implementation, or a specific way to encourage the drop target to start up a new thread?
Here is the meat of my DataObject implementation, if there's anything else that could be of use, I can provide.
STDMETHODIMP DataObject::QueryInterface(REFIID iid, LPVOID* ppvObject) {
if (ppvObject == nullptr) {
return E_INVALIDARG;
} else if (iid == IID_IUnknown) {
AddRef();
*ppvObject = reinterpret_cast<LPUNKNOWN>(this);
return S_OK;
} else if (iid == IID_IDataObject) {
AddRef();
*ppvObject = reinterpret_cast<LPDATAOBJECT>(this);
return S_OK;
} else if (iid == IID_IDataObjectAsyncCapability) {
AddRef();
*ppvObject = reinterpret_cast<IDataObjectAsyncCapability*>(this);
return S_OK;
}
*ppvObject = nullptr;
return E_NOINTERFACE;
}
STDMETHODIMP DataObject::GetData(LPFORMATETC queryFormat,
LPSTGMEDIUM outputStorage) {
if (queryFormat->cfFormat == CF_FILEDESCRIPTOR &&
(queryFormat->tymed & TYMED_HGLOBAL) && queryFormat->lindex <= 0 &&
queryFormat->dwAspect == DVASPECT_CONTENT) {
outputStorage->tymed = TYMED_HGLOBAL;
outputStorage->pUnkForRelease = nullptr;
outputStorage->hGlobal =
lockAndDuplicateHGLOBAL(this->groupDescriptorStorage.hGlobal);
return S_OK;
} else if (queryFormat->cfFormat == CF_FILECONTENTS &&
(queryFormat->tymed & TYMED_ISTREAM) &&
queryFormat->dwAspect == DVASPECT_CONTENT &&
queryFormat->lindex >= 0 &&
queryFormat->lindex < this->files.size()) {
// files is vector<pair<FILEDESCRIPTOR, STGMEDIUM>>
// where the STGMEDIUM is set to IStream
// Am I doing something wrong here?
auto file = this->files[queryFormat->lindex].second;
*outputStorage = file;
return S_OK;
}
return DV_E_FORMATETC;
}
STDMETHODIMP DataObject::QueryGetData(LPFORMATETC queryFormat) {
if (queryFormat->cfFormat == CF_FILEDESCRIPTOR ||
(queryFormat->cfFormat == CF_FILECONTENTS &&
(queryFormat->tymed & TYMED_HGLOBAL))) {
return S_OK;
}
return DATA_E_FORMATETC;
}
STDMETHODIMP DataObject::EnumFormatEtc(DWORD dwDirection,
LPENUMFORMATETC* ppEnumFormatEtc) {
if (dwDirection == DATADIR_GET) {
// basic implementation of IEnumFormatEtc (not mine)
return EnumFormat::Create(this->supportedFormats, this->numFormats,
ppEnumFormatEtc);
}
return E_INVALIDARG;
}
// Do the actual drag drop
// files is a list of STGMEDIUMS with IStreams. I'm using data that's readily available while
// implementing this, could that be somehow interfering and causing this problem in the first place?
DWORD dwEffect;
auto dataObject = new DataObject(groupDescriptorStorage, files);
auto dropSource = new DropSource();
// Do these DROPEFFECTS have an effect?
auto result = DoDragDrop(dataObject, dropSource,
DROPEFFECT_COPY | DROPEFFECT_MOVE, &dwEffect);
// By the time we get here, the data extraction would already have occured
// because it's happening inside DoDragDrop. of course, isAsyncOperation will be false
BOOL isInAsyncOperation;
dataObject->InOperation(&isInAsyncOperation);
I've followed the instructions from here as well as various forum posts I've found, but I can't seem to find anything about what to do when the drop target doesn't play ball.
I'm really stuck for ideas as to how to make this work. If the drop target isn't even querying for IDataObjectAsyncCapability then is it hopeless? Should I be returning something other than the file contents when it's first requested?