如何将日期时间转换为字符串？

 private void calendar_SelectedDatesChanged(object sender, SelectionChangedEventArgs e)
    {

        // ... Get reference.
        var calendar = sender as Calendar;

        // ... See if a date is selected.
        if (calendar.SelectedDate.HasValue)
        {

            DateTime date = calendar.SelectedDate.Value;
            this.Title = date.ToShortDateString();
            Stream stream = File.Open(Convert.ToString(date+".txt"), FileMode.Open);
            textbox.Visibility = Visibility.Visible;
            textbox.Text = "";
            stream.Close();

        }


    }
 private void calendar_SelectedDatesChanged(obj