class ConferenceRegistration{

    public string CampaignSource { get; private set; }
    public string FirstName { get; private set; }
    public string? MiddleName { get; private set; }
    public string LastName { get; private set; }
    public string Email { get; private set; }
    public DateTimeOffset CreatedOn { get; private set; } = DateTime.Now;

→ 모든 속성은 private set을 갖는다.

    public ConferenceRegistration(
        string firstName,
        string? middleName,
        string lastName,
        string email,
        string? campaignSource = null) {    → 널로 옵션을 나타낸다.
        FirstName = firstName;
        MiddleName = middleName;
        LastName = lastName;
        Email = email;
        CampaignSource = campaignSource ?? "organic";
    }
}