unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;
const WM_COMMNOTIFY = WM_USER + 12;
type
  TForm1 = class(TForm)
    Button1: TButton;
    Memo1: TMemo;
    procedure Button1Click(Sender: TObject);
  private
  Procedure MsgComm(Var Msg:Tmessage); Message WM_COMMNOTIFY;
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
begin
Memo1.Lines.Add('begining');
   Memo1.Lines.Add('The End');
   PostMessage(Form1.Handle,WM_COMMNOTIFY,0,0);

end;

Procedure TForm1.MsgComm(Var Msg:Tmessage);
begin
  Memo1.Lines.Add('User');
end;
end.

拿我这个直接贴PAS文件中试一下，对了加两上控件，memo1,button1；
我在DELPHI5下一点问题也没有
用sendmessage试一下

delphi不懂, 南蛮妈妈帮你顶