FormのClosingイベントの実行スレッドについて

//Form1コンストラクタの中でCloseを呼び出すデリゲートを作成しました
//delegate void CloseDeligate();
//CloseDeligate closeDeligate;
//closeDeligate = new CloseDeligate(Close);

void ExcuteBackup()
{
  TimeSpan restTimeSpan = new TimeSpan();
  while (stopEvent.WaitOne(1000, false) == false)
  {
    restTimeSpan = backupDateTime - DateTime.Now;
    UpdateStatusBar(restTimeSpan.ToString());
  }
  EnableStart(true);
}

void UpdateStatusBar(string text)
{
  if (InvokeRequired == true)
  {
    BeginInvoke(updateStatusBarDeligate, new object[] { text });
    return;
  }
  panel1.Text = text;
}

void EnableStart(bool enabled)
{
  if (InvokeRequired == true)
  {
    BeginInvoke(enableStartDeligate, new object[] { enabled });
    return;
  }
  startButton.Enabled = enabled;
  stopButton.Enabled = !enabled;
}

private void stopButton_Click(object sender, EventArgs e)
{
  stopEvent.Set();
}

private void OnClosing(object sender, CancelEventArgs e)
{
  if (backupThread != null)
  {
    if (backupThread.IsAlive == true)
    {
       stopEvent.Set();
       Thread.Sleep(1000);
       BeginInvoke(closeDeligate);
       e.Cancel = true;
       return;
    }
  }
}