tab 键的消息问题！

[ns001]

按TAB及任何键不执行 showmessage ，？

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, DB, ADODB, Grids, DBGrids;

type
TForm1 = class(TForm)
ComboBox1: TComboBox;
private
{ Private declarations }
public
{ Public declarations }
end;

type
TSACombo=class(TComboBox)
protected
procedure WMGetDlgCode(var Message: TWMGetDlgCode); message WM_GETDLGCODE;
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

procedure TSACombo.WMGetDlgCode(var Message: TWMGetDlgCode);
begin
Showmessage('1');
end;

end.