type
  TForm2 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  end;

  TTestThread = class(TThread)
  protected
    procedure Execute; override;
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure TForm2.Button1Click(Sender: TObject);
var MyThread : TTestThread;
begin
  MyThread:=TTestThread.Create(true);
  MyThread.FreeOnTerminate:=True;
  MyThread.Priority:=tpHighest;
  MyThread.Resume;
end;

{ TTestThread }

procedure TTestThread.Execute;
var len : integer;
begin
 len := Form2.Canvas.TextWidth('test');
 if (len=0) then
  Raise Exception.Create(SysErrorMessage(GetLastError));
end;

end.