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Drag and Drop

Post date: Mar 9, 2010 12:08:22 PM

Drag 'n' drop single or multiple files into your application to load the filename(s) into an array of pChar.

Add ShellAPI and Messages to your project's uses:

uses

    ShellAPI, Messages;

Add a method to your form that handles the WM_DROPFILES message.

For example, the following would be placed in the TForm1 declaration in the protected section:

protected

    procedure WMDropFiles(var Msg: TWMDropFiles); message WM_DROPFILES;

Drag 'n' drop are activated by calling:

DragAcceptFiles(Handle, true);

and deactivated by calling:

DragAcceptFiles(Handle, false);

It is normal to activate drag 'n' drop in the OnCreate event, and deactivate it in the OnClose or OnDestroy events:

procedure TForm1.FormCreate(Sender: TObject);

begin

  DragAcceptFiles(Handle, true);

end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);

begin

  DragAcceptFiles(Handle, false);

end;

This first procedure will enable you to drop files one at a time:

procedure TForm1.WMDropFiles(var Msg: TWMDropFiles);

var filename: PChar;

    length: LongWord;

begin

  length:= DragQueryFile(Msg.Drop, 0, NIL, 0);

  filename:= StrAlloc(length+1);

  DragQueryFile(Msg.Drop,0,filename,length+1);

  {do something with filename}

  Msg.Result:=0;

  inherited;

end;

This second procedure will enable you to drop multiple files at one time:

procedure TForm1.WMDropFiles(var Msg: TWMDropFiles);

var length, num, i: LongWord;

    filenames: array of PChar;

begin

  number:=DragQueryFile(Msg.Drop,$ffff,nil,0);

  SetLength(filenames,number);

  for i:=0 to n-1 do begin

    length:=DragQueryFile(Msg.Drop,i,nil,0);

    filenames[i]:=StrAlloc(length+1);

    DragQueryFile(Msg.Drop,i,filenames[i],length+1);

  end;

  {do whatever you want with the array filenames}

  Msg.Result:=0;

  inherited;

end;

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