    private void WritePersons()
    {
      string Filename = "Persons.txt";
      if (File.Exists(Filename))
        File.Delete(Filename);
      var s = new List<String>();
      foreach (Person p in Persons)
      {
        s.Add($"{p.FirstName}:{p.LastName}:{p.Gender.ToString()}");
      }
      File.AppendAllLines(Filename, s);
    }
    private void ReadPersons()
    {
      string Filename = "Persons.txt";
      if (File.Exists(Filename))
      {
        var lines = File.ReadAllLines(Filename);
        foreach (string line in lines)
        {
          var fields = line.Split(':');
          Person p;
          p.FirstName = fields[0];
          p.LastName = fields[1];
          p.Gender = (Gendertype)(Enum.Parse(typeof(Gendertype), fields[2]));
          Persons.Add(p);
        }
      }
    }