Drag and Drop from FileManager

Problem/Question/Abstract:

Drag and Drop from FileManager

Answer:

You need to use these 3 functions from the ShellApi:

DragAcceptFiles - registers whether a window accepts dropped files
DragQueryFile - retrieves the filename of a dropped file
DragFinish - releases memory allocated for dropping files


uses
  ShellApi;

..

procedure TForm1.FormCreate(Sender: TObject);
begin
  DragAcceptFiles(Form1.Handle, true);
  Application.OnMessage := AppMessage;
end;

{ message handler procedure }
// Delphi 1: type DWord = longint; .. FileIndex := -1;

procedure TForm1.AppMessage(var Msg: Tmsg; var Handled: Boolean);
const
  BufferLength: DWORD = 511;
var
  DroppedFilename: string;
  FileIndex: DWORD;
  NumDroppedFiles: DWORD;
  pDroppedFilename: array[0..511] of Char;
  DroppedFileLength: DWORD;
begin
  if Msg.message = WM_DROPFILES then
  begin
    FileIndex := $FFFFFFFF;
    NumDroppedFiles := DragQueryFile(Msg.WParam, FileIndex,
      pDroppedFilename, BufferLength);

    for FileIndex := 0 to (NumDroppedFiles - 1) do
    begin
      DroppedFileLength := DragQueryFile(Msg.WParam, FileIndex,
        pDroppedFilename, BufferLength);
      DroppedFilename := StrPas(pDroppedFilename);

      { process the file name you just received }
    end;
    DragFinish(Msg.WParam); { important to free memory }
    Handled := true;
  end;
end;

Notes:

When dropping files, the DroppedFilename is the complete path, not just the filename.ext
It is possible to drag and drop just a directory. So if you are expecting filenames, you have to check for existence yourself.
The filenames come in uppercased.