unit Unit1;

interface

uses
  Windows, Messages, Dialogs, SysUtils, Classes, Controls, Forms, ActiveX, ShlObj;

type
  TForm1 = class(TForm, IDropTarget)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure OpenFile(const dataObj: IDataObject);
  private
    { Private declarations }
    DropTargetHelper: IDropTargetHelper;
    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;

  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TForm1 }

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

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

function TForm1.DragOver(grfKeyState: Longint; pt: TPoint;
  var dwEffect: Longint): HResult;
begin
  dwEffect := DROPIMAGE_INVALID;
  Result:=DropTargetHelper.DragOver(pt, dwEffect);
end;

function TForm1.Drop(const dataObj: IDataObject; grfKeyState: Longint;
  pt: TPoint; var dwEffect: Longint): HResult;
begin
  dwEffect:=DROPEFFECT_COPY;
  Result:=DropTargetHelper.Drop(dataObj, pt, dwEffect);
  try
    OpenFile(dataObj);
  except
    on E:Exception do
      ShowMessage(E.Message);
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  OleInitialize(nil);
  CoCreateInstance(CLSID_DragDropHelper, nil, CLSCTX_INPROC_SERVER,
        IDropTargetHelper, DropTargetHelper);
  RegisterDragDrop(Handle, Self);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  RevokeDragDrop(Handle);
  OleUninitialize;
end;

procedure TForm1.OpenFile(const dataObj: IDataObject);
begin
  Raise Exception.Create('Test');
end;

end.