Author: Pablo Reyes 

This way you can drag and drop files to a specific control in a Delphi form

Answer:

Just create a project and add a ListBox component to Form1. 

1. First, a procedure to handle the message but without handling it. 

1   interface
2   
3   procedure WMDROPFILES(var Msg: TMessage);
4   
5   implementation
6   
7   procedure TForm1.WMDROPFILES(var Msg: TMessage);
8   var
9     pcFileName: PChar;
10    i, iSize, iFileCount: integer;
11  begin
12    pcFileName := ''; // to avoid compiler warning message
13    iFileCount := DragQueryFile(Msg.WParam, $FFFFFFFF, pcFileName, 255);
14    for i := 0 to iFileCount - 1 do
15    begin
16      iSize := DragQueryFile(Msg.wParam, 0, nil, 0) + 1;
17      pcFileName := StrAlloc(iSize);
18      DragQueryFile(Msg.WParam, i, pcFileName, iSize);
19      if FileExists(pcFileName) then
20        AddFile(pcFileName); // method to add each file
21      StrDispose(pcFileName);
22    end;
23    DragFinish(Msg.WParam);
24  end;


2. Second, a WindowProc method to replace ListBox1 WindowProc default method and a 
variable to store ListBox1 WindowProc default method. 

25  interface
26  
27  procedure LBWindowProc(var message: TMessage);
28  
29  implementation
30  
31  var
32    OldLBWindowProc: TWndMethod;
33  
34  procedure TForm1.LBWindowProc(var message: TMessage);
35  begin
36    if message.Msg = WM_DROPFILES then
37      WMDROPFILES(message); // handle WM_DROPFILES message
38    OldLBWindowProc(message);
39      // call default ListBox1 WindowProc method to handle all other messages
40  end;


3. In Form1 OnCreate event, initialize all. 
41  
42  procedure TForm1.FormCreate(Sender: TObject);
43  begin
44    OldLBWindowProc := ListBox1.WindowProc; // store defualt WindowProc
45    ListBox1.WindowProc := LBWindowProc; // replace default WindowProc
46    DragAcceptFiles(ListBox1.Handle, True); // now ListBox1 accept dropped files
47  end;


4. In Form1 OnDestroy event, uninitialize all. Not necesary but a good practice. 
48  
49  procedure TForm1.FormDestroy(Sender: TObject);
50  begin
51    ListBox1.WindowProc := OldLBWindowProc;
52    DragAcceptFiles(ListBox1.Handle, False);
53  end;


5. To complete source code, the AddFile method. 
54  
55  interface
56  
57  procedure AddFile(sFileName: string);
58  
59  implementation
60  
61  procedure TForm1.AddFile(sFileName: string);
62  begin
63    ListBox1.Items.Add(sFilename);
64  end;


6. Do not forget to add ShellAPI unit to the uses clause.