[twinBASIC] Explorer-style Drag-Drop Images with tB-defined x86+x64 interfaces

[fafalone]

A while back I posted this project:

[VB6] Register any control as a drop target that shows the Explorer drag image

Dragging from Explorer

Dragging from Firefox

I wanted to try my hand at using these interfaces in x64 apps. A 64-bit version of oleexp doesn't seem likely; there's seemingly insurmountable barriers to using midl with a project that needs to redefine things in the force-included headers, and the original types weren't preserved so every single interface would have to be manually reviewed and updated if it uses pointer types. But, twinBASIC lets you define interfaces as a native language feature. So I set about re-implementing all the necessary ones for this project:

[ InterfaceId ("4657278B-411B-11D2-839A-00C04FD918D0") ]
Interface IDropTargetHelper Extends stdole.IUnknown
    Sub DragEnter(ByVal hwndTarget As LongPtr, ByVal pDataObject As DragDropDemo.IDataObject, ppt As POINT, ByVal dwEffect As DROPEFFECTS)
    Sub DragLeave()
    Sub DragOver(ppt As POINT, ByVal dwEffect As DROPEFFECTS)
    Sub Drop(ByVal pDataObject As DragDropDemo.IDataObject, ppt As POINT, ByVal dwEffect As DROPEFFECTS)
    Sub Show(ByVal fShow As Long)
End Interface

[ InterfaceId ("DE5BF786-477A-11D2-839D-00C04FD918D0") ]
Interface IDragSourceHelper Extends stdole.IUnknown
    Sub InitializeFromBitmap(pshdi As SHDRAGIMAGE, pDataObject As DragDropDemo.IDataObject)
    Sub InitializeFromWindow(ByVal hwnd As LongPtr, ppt As POINT, pDataObject As DragDropDemo.IDataObject)
End Interface

[ InterfaceId ("83E07D0D-0C5F-4163-BF1A-60B274051E40") ]
Interface IDragSourceHelper2 Extends IDragSourceHelper
	Sub SetFlags(ByVal dwFlags As DSH_FLAGS)
End Interface

[ InterfaceId ("00000122-0000-0000-C000-000000000046") ]
Interface IDropTarget Extends stdole.IUnknown
    Sub DragEnter(ByVal pDataObject As DragDropDemo.IDataObject, ByVal grfKeyState As Long, ByVal pt As LongLong, dwEffect As DROPEFFECTS)
    Sub DragOver(ByVal grfKeyState As Long, ByVal pt As LongLong, pdwEffect As DROPEFFECTS)
    Sub DragLeave()
    Sub Drop(ByVal pDataObj As DragDropDemo.IDataObject, ByVal grfKeyState As Long, ByVal pt As LongLong, pdwEffect As DROPEFFECTS)
End Interface

[ InterfaceId ("00000121-0000-0000-C000-000000000046") ]
Interface IDropSource Extends stdole.IUnknown
    Sub QueryContinueDrag(ByVal fEscape As Long)
    Sub GiveFeedback(ByVal grfKeyState As Long)
End Interface

[ InterfaceId ("0000010f-0000-0000-C000-000000000046") ]
Interface IAdviseSink Extends stdole.IUnknown
	Sub OnDataChange(pFormatEtc As FORMATETC, pStgMed As STGMEDIUM)
    Sub OnViewChange(dwAspect As DVASPECT, ByVal lindex As Long)
    Sub OnRename(ByVal pmk As LongPtr) 'As IMoniker
    Sub OnSave()
    Sub OnClose()
End Interface

[ InterfaceId ("00000103-0000-0000-C000-000000000046") ]
Interface IEnumFormatETC Extends stdole.IUnknown
    Sub Next(ByVal celt As Long, rgelt As FORMATETC, pceltFetched As Long)
    Sub Skip(ByVal celt As Long)
    Sub Reset()
    Sub Clone(ppEnum As DragDropDemo.IEnumFormatETC)
End Interface

[ InterfaceId ("00000105-0000-0000-C000-000000000046") ]
Interface IEnumSTATDATA Extends stdole.IUnknown
    Sub Next(ByVal celt As Long, rgelt As STATDATA, pceltFetched As Long)
    Sub Skip(ByVal celt As Long)
    Sub Reset()
    Sub Clone(ppEnum As DragDropDemo.IEnumSTATDATA)
End Interface

[ InterfaceId ("0000010E-0000-0000-C000-000000000046") ]
Interface IDataObject Extends stdole.IUnknown
    Sub GetData(pFormatEtcIn As FORMATETC, pMedium As STGMEDIUM)
    Sub GetDataHere(pFormatEtc As FORMATETC, pMedium As STGMEDIUM)
    Sub QueryGetData(pFormatEtc As FORMATETC)
    Sub GetCanonicalFormatEtc(pFormatEtcIn As FORMATETC, pFormatEtcOut As FORMATETC)
    Sub SetData(pFormatEtc As FORMATETC, pMedium As STGMEDIUM, ByVal fRelease As Long)
    Function EnumFormatEtc(ByVal dwDirection As DATADIR) As DragDropDemo.IEnumFormatETC
    Sub DAdvise(pFormatEtc As FORMATETC, ByVal advf As ADVF, pAdvSink As DragDropDemo.IAdviseSink)
    Sub DUnadvise(ByVal dwConnection As Long)
    Function EnumDAdvise() As DragDropDemo.IEnumSTATDATA
End Interface

'CLSID_DragDropHelper 
[ CoClassId ("4657278A-411B-11D2-839A-00C04FD918D0") ]
CoClass DragDropHelper
	 [ Default ] Interface IDropTargetHelper
	Interface IDragSourceHelper
	Interface IDragSourceHelper2
End CoClass

I used functions for some to preserve as much compatibility as possible with oleexp-using code.

There were a couple tricky things here... it's always been an odd interface. Normally, UDTs as [in] are ByRef; MSDN lists the x,y coords as POINT, and we had to expand that to 2 ByVal Long's in x86. It's even weirder in x64; we have to use a single member, but it's also ByVal... so it's 8 bytes but not *actually* a pointer, so it's handled like this:

Private Sub IDropTarget_DragEnter(ByVal pDataObj As DragDropDemo.IDataObject, ByVal grfKeyState As Long, ByVal pt As LongLong, pdwEffect As DragDropDemo.DROPEFFECTS)
    Dim ptt As DragDropDemo.POINT
    CopyMemory ptt, pt, LenB(ptt)

Since LongLong is available on x86 too, we can still use one definition, if a slightly more complicated CopyMemory call is used for both.

Also, lots of things are fully qualified as there's currently an issue with conflicting types in the WinNativeForms package that are exposed to users.

But besides those quirks, the code works on x64 without major change, just updating to LongPtr where needed, and replacing the PictureBox with a Frame until that's available.

Et voilà:


Requirements
Windows Vista or newer
twinBASIC Beta 167 or newer

Thanks to twinBASIC developer Wayne Phillips for his help getting this working, and of course for the continuing great work on twinBASIC itself

Update: I've added DragDropDemoWDL.twinproj that uses WinDevLib for interfaces/APIs. WDL uses proper ByVal UDTs instead of hacks and has minor API signature differences due to stricter standards for SDK compatibility. The original project is left up as an intro to defining interfaces yourself in tB.

(Project update: v5 adds force registration option to Attach to unregister an existing drop target. Primarily to sync with the VB6 version, as tB doesn't currently have the bug where it registers dragdrop for a form with windowless UC where neither enable dragdrop.

(Project update: v4 suppresses errors from TotalCommander and others in the DragDropHelper object since file names can usually still be retrieved.

(Project update: V2.1 updates to use Coclass syntax, requires twinBASIC Beta 167 or newer)

Project update: V2 unifies the declarations to use LongLong for the POINT on both x86 and x64, and corrects the definition of IEnumSTATDATA.Next.

Project update: Updated to use new Coclass syntax

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Download latest version on GitHub.