Hierarchy of value objects - full code

full.cs

public class Person : Entity
{
    public virtual string Name { get; set; }

    private readonly DocumentContainer _document;
    public virtual Document Document
    {
        get => _document.Document;
        set => _document.Document = value;
    }
}

public abstract class Document : ValueObject
{
}

public class IdentityCard : Document
{
    public DateTime IssueDate { get; }

    public IdentityCard(DateTime issueDate)
    {
        IssueDate = issueDate;
    }

    protected override IEnumerable<object> GetEqualityComponents()
    {
        yield return IssueDate;
    }
}

public class Passport : Document
{
    public string SerialNumber { get; }

    public Passport(string serialNumber)
    {
        SerialNumber = serialNumber;
    }

    protected override IEnumerable<object> GetEqualityComponents()
    {
        yield return SerialNumber;
    }
}

public class DocumentContainer
{
    private DocumentType _type;
    private DateTime? _issueDate;
    private string _serialNumber;

    public Document Document
    {
        get
        {
            switch (_type)
            {
                case DocumentType.IdentityCard:
                    return new IdentityCard(_issueDate.Value);

                case DocumentType.Passport:
                    return new Passport(_serialNumber);

                default:
                    throw new ArgumentOutOfRangeException();
            }
        }
        set
        {
            switch (value)
            {
                case IdentityCard identityCard:
                    _issueDate = identityCard.IssueDate;
                    _serialNumber = null;
                    _type = DocumentType.IdentityCard;
                    break;

                case Passport passport:
                    _issueDate = null;
                    _serialNumber = passport.SerialNumber;
                    _type = DocumentType.Passport;
                    break;
            }
        }
    }
}

public enum DocumentType
{
    IdentityCard = 1,
    Passport = 2
}

public class PersonMap : ClassMap<Person>
{
    public PersonMap()
    {
        Id(x => x.Id);
        Map(x => x.Name);

        Component<DocumentContainer>(Reveal.Member<Person>("_document"), y =>
        {
            y.Map(Reveal.Member<DocumentContainer>("_type"), "DocumentType").CustomType<int>().Access.Field();
            y.Map(Reveal.Member<DocumentContainer>("_issueDate"), "DocumentIssueDate").Access.Field().Nullable();
            y.Map(Reveal.Member<DocumentContainer>("_serialNumber"), "DocumentSerialNumber").Access.Field().Nullable();
        }).Access.Field();
    }
}