Identifier.cs

using System;

namespace Identity.Models
{
    public abstract class Identifier
    {
        public DateTime CreationTime { get; set; }
    }

    public abstract class ContactableIdentifier : Identifier
    {
        public bool Verified { get; set; }
    }

    public class UsernameIdentifier : Identifier
    {
        public UsernameIdentifier(string username)
        {
            Username = username;
        }

        public string Username { get; set; }

        public override string ToString() => Username;
    }

    public class EmailIdentifier : ContactableIdentifier
    {
        public EmailIdentifier(string email)
        {
            Email = email;
        }

        public string Email { get; set; }

        public override string ToString() => Email;
    }
}