Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

{ 
  This is useful if you want to send a message from your DLL back 
  to the calling application. 
}

 const
   MY_MESSAGE = WM_USER + 4242;

   type
   TForm1 = class(TForm)
     Button1: TButton;
     procedure Button1Click(Sender: TObject);
     // Handler that receive the Message 
    procedure MessageReceiver(var msg: TMessage); message MY_MESSAGE;
   end;

 var
   Form1: TForm1;

 implementation

 {$R *.DFM}

 procedure TForm1.Button1Click(Sender: TObject);
 var
   txt: string;
 begin
   txt := 'Hello World';
   SendMessage(Form1.Handle, MY_MESSAGE, 0, DWORD(PChar(txt)));
 end;

 // To receive this custom Message that is addressed to form1.handle 
// you need a message handler. 

procedure TForm1.MessageReceiver(var msg: TMessage);
 var
   txt: PChar;
 begin
   txt := PChar(msg.lParam);
   msg.Result := 1;
   ShowMessage(txt);
 end;


 end.