Accept dropped files from the explorer

Problem/Question/Abstract:

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.

interface

procedure WMDROPFILES(var Msg: TMessage);

implementation

procedure TForm1.WMDROPFILES(var Msg: TMessage);
var
  pcFileName: PChar;
  i, iSize, iFileCount: integer;
begin
  pcFileName := ''; // to avoid compiler warning message
  iFileCount := DragQueryFile(Msg.WParam, $FFFFFFFF, pcFileName, 255);
  for i := 0 to iFileCount - 1 do
  begin
    iSize := DragQueryFile(Msg.wParam, 0, nil, 0) + 1;
    pcFileName := StrAlloc(iSize);
    DragQueryFile(Msg.WParam, i, pcFileName, iSize);
    if FileExists(pcFileName) then
      AddFile(pcFileName); // method to add each file
    StrDispose(pcFileName);
  end;
  DragFinish(Msg.WParam);
end;

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

interface

procedure LBWindowProc(var Message: TMessage);

implementation

var
  OldLBWindowProc: TWndMethod;

procedure TForm1.LBWindowProc(var Message: TMessage);
begin
  if Message.Msg = WM_DROPFILES then
    WMDROPFILES(Message); // handle WM_DROPFILES message
  OldLBWindowProc(Message);
    // call default ListBox1 WindowProc method to handle all other messages
end;

3. In Form1 OnCreate event, initialize all.

procedure TForm1.FormCreate(Sender: TObject);
begin
  OldLBWindowProc := ListBox1.WindowProc; // store defualt WindowProc
  ListBox1.WindowProc := LBWindowProc; // replace default WindowProc
  DragAcceptFiles(ListBox1.Handle, True); // now ListBox1 accept dropped files
end;

4. In Form1 OnDestroy event, uninitialize all. Not necesary but a good practice.

procedure TForm1.FormDestroy(Sender: TObject);
begin
  ListBox1.WindowProc := OldLBWindowProc;
  DragAcceptFiles(ListBox1.Handle, False);
end;

5. To complete source code, the AddFile method.

interface

procedure AddFile(sFileName: string);

implementation

procedure TForm1.AddFile(sFileName: string);
begin
  ListBox1.Items.Add(sFilename);
end;

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


Component Download: DroppedFiles.zip