Author: William Gerbert 

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


1   uses
2     ShellApi;
3   
4   ..
5   
6   procedure TForm1.FormCreate(Sender: TObject);
7   begin
8     DragAcceptFiles(Form1.Handle, true);
9     Application.OnMessage := AppMessage;
10  end;
11  
12  { message handler procedure }
13  // Delphi 1: type DWord = longint; .. FileIndex := -1;
14  
15  procedure TForm1.AppMessage(var Msg: Tmsg; var Handled: Boolean);
16  const
17    BufferLength: DWORD = 511;
18  var
19    DroppedFilename: string;
20    FileIndex: DWORD;
21    NumDroppedFiles: DWORD;
22    pDroppedFilename: array[0..511] of Char;
23    DroppedFileLength: DWORD;
24  begin
25    if Msg.message = WM_DROPFILES then
26    begin
27      FileIndex := $FFFFFFFF;
28      NumDroppedFiles := DragQueryFile(Msg.WParam, FileIndex,
29        pDroppedFilename, BufferLength);
30  
31      for FileIndex := 0 to (NumDroppedFiles - 1) do
32      begin
33        DroppedFileLength := DragQueryFile(Msg.WParam, FileIndex,
34          pDroppedFilename, BufferLength);
35        DroppedFilename := StrPas(pDroppedFilename);
36  
37        { process the file name you just received }
38      end;
39      DragFinish(Msg.WParam); { important to free memory }
40      Handled := true;
41    end;
42  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.