Hi,

I must be making a very stupid mistake: I am unable to get IDropTarget
to work. Always when calling RegisterDragDrop I get an (OLE) error
stating that there is too little memory for this operation.

The complete code I am using can be found below. There is no type
library, just a plain new application with the below modifications.

What am I doing wrong? I am just looking for the most simplistic
example that lets me RegisterDragDrop something (later implementing
IDropSource).

TIA
Stefan

*******************

uses
  ActiveX, ComObj, ComServ,
  .... <standard >;

type
  TForm1 = class(TForm, IDropTarget)
    Button1: TButton;
    Button2: TButton;
    procedure Button2Click(Sender: TObject);
    procedure Button1Click(Sender: TObject);

  public
    function IDropTarget.DragOver = _DragOver; // kill compiler
warning

    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;

  end;

{ implementation section - just stubbed to get everything to compile }

procedure TForm1.Button1Click(Sender: TObject);
begin
  OleCheck( RegisterDragDrop(Self.Handle, Self) );
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  OleCheck( RevokeDragDrop(Self.Handle) );
end;

{ Interface definition }

function TForm1.DragEnter(const dataObj: IDataObject; grfKeyState:
Longint;
  pt: TPoint; var dwEffect: Longint): HResult;
begin
  Result := S_OK;
end;

{ _DragOver aliased to IDropTarget.DragOver }

function TForm1._DragOver(grfKeyState: Longint; pt: TPoint;
  var dwEffect: Longint): HResult;
begin
  Result := S_OK;
end;

function TForm1.DragLeave: HResult;
begin
  Result := S_OK;
end;

function TForm1.Drop(const dataObj: IDataObject; grfKeyState: Longint;
pt: TPoint;
  var dwEffect: Longint): HResult;
begin
  Result := S_OK;
end;

--
mailto:Stefan.Hoffmeister (at) Uni-Passau.de
http://kakadu.rz.uni-passau.de/~w4hoff01/
    PGP public key at homepage

Hello,

Off of the top of my head, I think you need to first lock
the drop target class using the CoLockObjectExternal API
before you call calling RegisterDragDrop.

And don't forget to unlock your object using the same
CoLockObjectExternal API when you're done with the IDropTarget
implementor class.
--
Binh Ly
Brickhouse Data Systems, Inc
http://www.brickhouse.com

Stefan Hoffmeister <no.spam.ple...@see.signature> wrote in article
<34b75ad1.15734...@forums.borland.com>...

  OleCheck( CoLockObjectExternal(Self, true, false) ); // lock

and the unlock at the appropriate places, after having added IUnknown
to the form's class:

  TForm1 = class(TForm, IUnknown, IDropTarget)

Strangely I am now crashing with a nil pointer access somewhere in the
access to the IUnknown interface: a simple assignment "someinterface
:= self" at least crashes...

Any ideas what I am missing here?

Stefan
--
mailto:Stefan.Hoffmeister (at) Uni-Passau.de
http://kakadu.rz.uni-passau.de/~w4hoff01/
    PGP public key at homepage

Remember to do OLEInitialise(nil) in your units initialization
and OleUnInitialize in the finalization. Once these are in, everything
should work just fine.

The CoLockObjectExternal should go in the form's OnCreate method
and the corresponding CoLockObjectExternal (release) in the
OnDestroy.

Cheers,

            _  _______________________________  _
           / )|        David Brock            |( \
          / / |      dbr...@cqm.co.uk         | \ \
        _( (_ |  _  Paisley - Scotland     _  | _) )_
       (((\ \)|_/ )_______________________( \_|(/ /)))
       (\\\\ \_/ /                         \ \_/ ////)
        \       /                           \       /
         \    _/                             \_    /
         /   /                                 \   \

You can't add IUnknown to TForm without implementing it, unfortunately.
There's an "it would be a bug if it weren't a design decision" issue
with TForm (TCustomForm actually) -- it implements _some_ COM stuff, to
(I think) support ActiveForms.  you didn't get an error for failing to
implement the IUnknown methods in your TForm1 class because of that --
but you've got to re-implement them because you need them to work
differently than the ones in TCustomForm.

An alternative is to implement something you could call TInterfacedForm,
which would be     TInterfacedForm = class(TForm, IUnknown)
and which has Addref/Release/QI implemented, then descend from that.

I'm not sure I know why you added IUnknown to the form's class; that's
what caused the trouble.  Did you have a compile error if you didn't do
so?

!^NavFont02F02A20004HIA31C60

Is there sample code anywhere that shows how to do it right? Currently
I return E_NOINTERFACE and a refcount of 0 each time (thanks to Edwin
Hagen for the code) - anything wrong with that?

All in all my problem boiled down to:

1) I did not call OleInitialize and OleUninitialize
   --> caused OleSysError -> "Not enough storage is available..."

2) I did *not* re-implement the IUnknown interface as I thought that
it already had been implemented; it turned out that the existing
implementation is useless (not ready-to-use).
   --> caused AV to nil pointer

--
mailto:Stefan.Hoffmeister (at) Uni-Passau.de
http://kakadu.rz.uni-passau.de/~w4hoff01/
    PGP public key at homepage

Stefan,

  Are you trying to implement IDropTarget so
that you can drop files onto your form (or
a component on your form) and process them
in some manner (open, add to a list or treeview)?

  If so, save yourself some valuable time and
go to the Delphi Super Page and do a search
for the author "Angus Johnson" or his contribution,
"dndfiles.zip".

  Or, you might try this directly:

  http://SunSITE.icm.edu.pl/delphi/ftp/d20free/dndfiles.zip

There are two, very well-written components, one
implementing the drag source, and the other the drag target.
Properties for the important functionality allow you
to write the event handling.  There's a demo with
it that shows everything.  A very well-done contribution.

Paul Furbacher