/*用户撰写邮件User to Doctor*/
 public string MessageCompose(string userID, string password, string receiverID, string text) {
     UserService userService = new UserService();
     UserInfo userInfo = userService.Login(userID, password);
     if (userInfo.ErrorMessage == null) {
         return userService.MessageCompose(receiverID, text);
     }
     else {
         return userInfo.ErrorMessage;
     }
 }