Drag Drop via Ole

[direktor05 2]

Hello,

 

I want to do drag drop from Windows Explorer to my app via OLE interface IDropTarget. Question is how do I detect if OLE communication is working well. For now RegisterDragDrop function is working fine. But I can't get a call to DragEnter function when dragging starts. How can I check that OLE is working fine and that dragging somehow starts? But why I can't get DragEnter function to run? Here is some nice theory: https://delphidabbler.com/articles/article-24

 

I can't get any of this functions to work or be called. What is wrong? And how do I check if OLE can detect drag-drop in the first place?

function DragEnter(const dataObj: IDataObject; grfKeyState: Longint;  pt: TPoint; var dwEffect: Longint): HResult; stdcall;

function DragOver(grfKeyState: Longint; pt: TPoint; var dwEffect: Longint): HResult; stdcall;

function DragLeave: HResult; stdcall;

function Drop(const dataObj: IDataObject; grfKeyState: Longint; pt: TPoint; var dwEffect: Longint): HResult; stdcall;

[direktor05 2]

https://learn.microsoft.com/en-us/windows/win32/shell/dragdrop

 

I'm stopped at point 1 or 2.

[direktor05 2]

RegisterDragDrop(Handle, Handler);

 

Handle: 722688

Handler: TFileDropTarget($23747B02490) as IDropTarget

 

So I have a pointer to IDropTarget, but it does not call IDropTarget.DropEnter function when I start the drag operation. Why not? And how can I check this?

 

[direktor05 2]

RegisterDropTarget(TFileDropTarget.Create(FileDragEnter, FileDragOver, FileDragDrop, FileDragLeave));

[direktor05 2]

https://www.swissdelphicenter.ch/en/showcode.php?id=1627

[direktor05 2]

But there still is a question why my DragEnter code is not executed? And how can I find the answer?

[direktor05 2]

procedure TForm1.Create(Sender: TObject);
begin
  RegisterDropTarget(TFileDropTarget.Create(FileDragEnter, FileDragOver, FileDragDrop, FileDragLeave));
end;

procedure TForm1.RegisterDropTarget(Handler: IDropTarget);
begin
  RegisterDragDrop(Handle, Handler);
  FDragDropHandle := Handle;
end;

procedure TForm1.FileDragEnter(Sender: TObject; FileList: TStrings; Point: TPoint; Shift: TShiftState; var Action: TFileDropAction);
begin
  IT NEVER ENTERS HERE
  FDragFromWindows := True;
  Point := ScreenToClient(Point);
  UpdateFileDragAction(FileList, Point, Action);
end;

[direktor05 2]

How can I check if OLE is called? CoInitialize works OK. RegisterDragDrop also OK. There is no error anywhere but the thing does not work. How can I test a call to OLE dll?

[Anders Melander 1784]

Is there are reason why don't just use one of these?

• https://github.com/landrix/The-Drag-and-Drop-Component-Suite-for-Delphi
• https://github.com/andersmelander/Drag-and-Drop-Component-Suite

If you don't want to use a ready-made library then I suggest you at least use the DragTargetAnalyzer example application in the above library to diagnose your target.

[Ian Branch 127]

FWIW I can recommend the landrix suite, it is based on anders' suite.

[Remy Lebeau 1398]

7 hours ago, direktor05 said:

procedure TForm1.Create(Sender: TObject);
begin
  RegisterDropTarget(TFileDropTarget.Create(FileDragEnter, FileDragOver, FileDragDrop, FileDragLeave));
end;

The Form's OnCreate event is not the best place to register the drop target.  If your Form's window is ever recreated at runtime, you will lose your registration.  Instead, override the Form's virtual CreateWnd() method and do the registration there instead.

Edited October 4 by Remy Lebeau

[direktor05 2]

Anders your component probably does diagnostic according to your component functions not in general. I need a solution how do you check if a procedure of function is executed/called inside OLE/ActiveX dll, better yet specifically inside shell drag drop dll? Any hero here?

[Remy Lebeau 1398]

21 minutes ago, direktor05 said:

how do you check if a procedure of function is executed/called inside OLE/ActiveX dll, better yet specifically inside shell drag drop dll?

You don't.  The drag code is running inside of the Windows Explorer process.  If it is not invoking the IDropTarget object inside of your process, it means the call is being blocked outside of your process, most likely by UAC/UIPI if your app is running elevated.  Per the comments on your StackOverflow question on this topic:

Quote

Note that Windows doesn't support drag and drop between programs running at different integrity levels since Wndows Vista. Programs running with Administrator prviledge will not be a drop target for stuff dragged from programs not running with Administrator priviledge.

– Brian

Quote

It can work if you handle WM_DROPFILES (DragAcceptFiles()) instead of IDropTarget (RegisterDragDrop()), and then use ChangeWindowMessageFilter/Ex() to allow the WM_DROPFILES, WM_COPYDATA, and (undocumented) WM_COPYGLOBALDATA (0x0049) messages. ChangeWindowMessageFilter/Ex() doesn't work with IDropTarget, though. The alternative is to either 1) enable UIAccess=true for your app (this has very restrictive requirements, though), or 2) run a non-elevated process to accept the drag and then IPC the data into your elevated app.

- Remy Lebeau

 

Edited October 5 by Remy Lebeau

[direktor05 2]

Remy you are genius!

[Anders Melander 1784]

On 10/5/2024 at 10:48 PM, direktor05 said:

Anders your component probably does diagnostic according to your component functions not in general.

No; It works with all drop targets.

 

And if you look at the WM_DROPFILES example there's also an example of working around the UAC restrictions mentioned by Remy:

https://github.com/andersmelander/Drag-and-Drop-Component-Suite/blob/de46eb2b804c33921809504d086285da11d4e3e5/Source/DropTarget.pas#L298

[direktor05 2]

14 hours ago, Anders Melander said:

No; It works with all drop targets.

 

And if you look at the WM_DROPFILES example there's also an example of working around the UAC restrictions mentioned by Remy:

https://github.com/andersmelander/Drag-and-Drop-Component-Suite/blob/de46eb2b804c33921809504d086285da11d4e3e5/Source/DropTarget.pas#L298

Is this component made only for Delphi up to 2011? How about Delphi 11?

[direktor05 2]

Not very professional! Doesn't work at all!

[direktor05 2]

14 hours ago, Anders Melander said:

No; It works with all drop targets.

 

And if you look at the WM_DROPFILES example there's also an example of working around the UAC restrictions mentioned by Remy:

https://github.com/andersmelander/Drag-and-Drop-Component-Suite/blob/de46eb2b804c33921809504d086285da11d4e3e5/Source/DropTarget.pas#L298

You could also tell me your idea of "example of working around the UAC restrictions " so I don't have to go through your non-working code.

 

[direktor05 2]

In my code the file list is not filled:

 

FileName := PWideChar(Cardinal(DropFiles) + DropFiles.pFiles);

 

where DropFiles comes from WinApi.ShlObj - which means the file list is not being transferred from Windows on IDropTarget.DragEnter

 

IDropTarget.DragEnter is called but the file list is empty.

[Anders Melander 1784]

1 minute ago, direktor05 said:

You could also tell me your idea of "example of working around the UAC restrictions " so I don't have to go through your non-working code.

Bite me

[direktor05 2]

ha ha, ok I'll bite you

 

[Remy Lebeau 1398]

1 hour ago, direktor05 said:

FileName := PWideChar(Cardinal(DropFiles) + DropFiles.pFiles);

That code risks pointer truncation in a 64bit build. The integer typecast should be using NativeUInt or UIntPtr  instead. Or, use pointer arithmetic via PByte instead.

 

Also, the file list is a double-null terminated list, so make sure you account for that in case multiple files are dragged. Also, depending on the source, the file names may be using Ansi or Unicode characters, so pay attention to the DropFiles.fWide field. 

Edited October 7 by Remy Lebeau

[direktor05 2]

16 hours ago, Remy Lebeau said:

That code risks pointer truncation in a 64bit build. The integer typecast should be using NativeUInt or UIntPtr  instead. Or, use pointer arithmetic via PByte instead.

 

Also, the file list is a double-null terminated list, so make sure you account for that in case multiple files are dragged. Also, depending on the source, the file names may be using Ansi or Unicode characters, so pay attention to the DropFiles.fWide field. 

REMY YOU SOLVED MY PROBLEM! THANKS.

[Jirka52 2]

You can check in your code these problems with FastMM4. I had the same problem.

Look at setting of Fastmm4:

https://www.atozed.com/2021/07/detecting-memory-leaks-in-delphi-applications-using-fastmm-4/

It is from blog of @dummzeuch

https://blog.dummzeuch.de/2024/09/26/you-should-be-using-the-full-fastmm/

Try to run this code in 64 bit with Fastmm4 and you will probably see exception.