Code:

unit Main;

interface

uses

Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

StdCtrls;

type

TMainForm = class(TForm)

  Button1: TButton;

  procedure Button1Click(Sender: TObject);

private

  { Private declarations }

public

  { Public declarations }

end;

var

MainForm: TMainForm;

implementation

{$R *.DFM}

{ NOTE: Change GlobalStr from var to threadvar to see difference }

var

//threadvar

GlobalStr: string;

type

TTLSThread = class(TThread)

private

  FNewStr: string;

protected

  procedure Execute; override;

public

  constructor Create(const ANewStr: string);

end;

procedure SetShowStr(const S: string);

begin

if S = '' then

  MessageBox(0, PChar(GlobalStr), 'The string is...', MB_OK)

else

  GlobalStr := S;

end;

constructor TTLSThread.Create(const ANewStr: string);

begin

FNewStr := ANewStr;

inherited Create(False);

end;

procedure TTLSThread.Execute;

begin

FreeOnTerminate := True;

SetShowStr(FNewStr);

SetShowStr('');

end;

procedure TMainForm.Button1Click(Sender: TObject);

begin

SetShowStr('Hello world');

SetShowStr('');

TTLSThread.Create('Dilbert');

Sleep(100);

SetShowStr('');

end;

end.