解析为DateTime类型的字符串

我正试图将DateTime和int属性转换为foreach循环中的字符串。我需要将存储在对象属性中的信息放入变量中，然后使用该信息使用StreamWriter写入文本文件。我不太确定我做错了什么。Parse为红色下划线，显示错误信息-

错误2 'string'不包含'Parse'的定义

这是我的代码-

 public bool Save()
        {
            string appointStart;
            string appointLength;
            string appointDescription;
            foreach (Appointment appointment in appointments)
            {
                appointStart = string.Parse(appointment.Start);
                appointLength = string.Parse(appointment.Length);
                appointDescription = appointment.DisplayableDescription;
            }
            return true;
        }

谢谢

foreach (Appointment appointment in appointments)
{
  appointStart = appointment.Start.ToString();
  appointLength = appointment.Length.ToString();
  appointDescription = appointment.DisplayableDescription;
}