Cannot make DragAcceptFiles work when using a dialog box

Question

I am using Delphi to code a simple app that needs to accept dropped directory names onto it. Though I am using Delphi, this program is straight Windows API (no VCL, no object oriented stuff) just as it would be in plain C.

I had some code that already worked using a normal window (as created by CreateWindowEx). When I converted that code to use a dialog box, the drag and drop functionality broke and I cannot figure out why.

This is (part of) the code I had that worked fine:

function WndProc (Wnd : hWnd; Msg, wParam, lParam : DWORD) : DWORD; stdcall;
  { main application/window handler function                                  }
const
  DROPPED_FILE_COUNT  = -1;

var
  FileCnt     : integer;                            { number of files dropped }

  Filename    : packed array[0..MAX_PATH] of char;  { filename buffer         }
  DropInfo    : HDROP absolute wParam;              { wParam points to Drop...}

  I           : integer;                            { for loop                }

begin
  WndProc := 0;

  { let the Windows default handler take care of everything                   }

  case msg of
    WM_CREATE:
       begin
         InitCommonControls;

         if CreateWindowEx(WS_EX_CLIENTEDGE or WS_EX_RIGHTSCROLLBAR,
                           'LISTBOX',
                           nil,
                           WS_CHILD        or WS_HSCROLL      or WS_VSCROLL or
                           WS_CLIPSIBLINGS or WS_CLIPCHILDREN or
                           WS_VISIBLE      or LBS_NOINTEGRALHEIGHT,
                           0,
                           0,
                           0,
                           0,
                           Wnd,
                           IDC_LISTBOX,
                           hInstance,
                           nil) = 0 then
         begin
           MessageBox(Wnd,
                      'Couldn''t create the listbox', 'Main Window', MB_OK);

           WndProc := -1;
         end;

         { let Windows know that we accept files being dragged over our client}
         { area.                                                              }

         DragAcceptFiles(Wnd, TRUE);

         { tell the listbox to use a nice font                                }

         SendMessage(GetDlgItem(Wnd, IDC_LISTBOX),
                     WM_SETFONT, GetStockObject(DEFAULT_GUI_FONT), 0);

         exit;
       end; { WM_CREATE }

    WM_DROPFILES:
       begin
         { one or more files have been dropped on us!                         }

         FileCnt := DragQueryFile(DropInfo, DROPPED_FILE_COUNT, nil, 0);

         for I := 0 to FileCnt - 1 do
           begin
             { get the dropped files names and add them to the listbox        }

             DragQueryFile(DropInfo, I, Filename, sizeof(Filename));

             ListBox_AddString(GetDlgItem(Wnd, IDC_LISTBOX), Filename);
           end;

         { tell Windows that we are done grabbing the dropped files           }

         DragFinish(DropInfo);

         exit;
       end; { WM_DROPFILES }

    ... followed by other stuff that has nothing to do with drag and drop ... 

I converted that to this:

procedure ExecWM_INITDIALOG(wnd : hWnd);
begin
  { ensure that the listbox isn't accepting dropped files }

  DragAcceptFiles(GetDlgItem(Wnd, IDC_DIRECTORIES), FALSE);

  { ensure the main dialog window accept dropped files }

  DragAcceptFiles(Wnd, TRUE);

  { load the current values in the registry                                   }

  LoadRegistrySettings(RegistryKey, RegistrySettings, RegistrySettingsCount)
end;

procedure ExecWM_DROPFILES(Wnd : hWnd; wParam : word; lParam : longint);
const
  DROPPED_FILE_COUNT  = -1;

var
  FileCnt     : integer;                            { number of files dropped }

  Filename    : packed array[0..MAX_PATH] of char;  { filename buffer         }
  DropInfo    : HDROP absolute wParam;              { wParam points to Drop...}

  I           : integer;                            { for loop                }
begin
  { one or more files have been dropped on us!                         }

  { -->>> The problem seems to show up at this statement:              }
  {        the DropInfo (wParam) has a value that seems too low ($20)  }
  {        when using the code that works the value is much larger     }

  FileCnt := DragQueryFile(DropInfo, DROPPED_FILE_COUNT, nil, 0);

  for I := 0 to FileCnt - 1 do
    begin
      { get the dropped files names and add them to the listbox        }

      DragQueryFile(DropInfo, I, Filename, sizeof(Filename));

      ListBox_AddString(GetDlgItem(Wnd, IDC_DIRECTORIES), Filename);
    end;

  { tell Windows that we are done grabbing the dropped files           }

  DragFinish(DropInfo);
end;

function BackupConfigProc(wnd      : hWnd;
                          msg      : word;
                          wParam   : word;
                          lParam   : longint) : bool; stdcall;
begin
  BackupConfigProc := FALSE;            { default return value                }

  case msg of
    WM_COMMAND:
    begin
      ExecWM_COMMAND (wnd, wParam, lParam);
      BackupConfigProc := TRUE;
    end;

    WM_DROPFILES:
    begin
      ExecWM_DROPFILES(Wnd, wParam, lParam);
      //BackupConfigProc := TRUE;    { this was a shot in the dark }
    end;

    WM_INITDIALOG:
       begin
         ExecWM_INITDIALOG (wnd);
         BackupConfigProc := TRUE;
       end;

    WM_CLOSE:
       begin
         EndDialog(wnd, 0);     { and return the default FALSE                }
       end;
  end;
end;

begin
  DialogBox(hInstance, 'BackupConfigDlg', 0, @BackupConfigProc);
end.

It looks like the value of the DropInfo (wParam) received by the dialog box is not valid (as it is much lower in value than the one received by the non dialog code that works).

I will be grateful to all who can shed some light as to why the Dialog version does not work and what needs to be done to make it work.

Thank you,

John.

Answer 1

procedure ExecWM_DROPFILES(Wnd : hWnd; wParam : word; lParam : longint);

wParam should not be typed word but Windows.WPARAM which is actually a long int (on Win32).