Author: Lou Adler 

How to use PostMessage to post a string instead of an integer 

Answer:

1   unit Unit1;
2   
3   interface
4   
5   uses
6     Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
7     Dialogs, StdCtrls;
8   
9   type
10    TForm1 = class(TForm)
11      Button1: TButton;
12      procedure Button1Click(Sender: TObject);
13    private
14      procedure wmUser(var Msg: TMessage); message WM_USER;
15    public
16    end;
17  
18  var
19    Form1: TForm1;
20  
21  implementation
22  
23  {$R *.dfm}
24  
25  procedure TForm1.Button1Click(Sender: TObject);
26  var
27    lP: LPARAM;
28  begin
29    lP := 0; {VERY important}
30    string(lP) := Caption + ' Whatever';
31    PostMessage(Handle, WM_USER, 0, lP);
32  end;
33  
34  procedure TForm1.wmUser(var Msg: TMessage);
35  begin
36    Caption := string(Msg.LParam);
37    string(Msg.LParam) := ''; {VERY important}
38  end;
39  
40  end.