Customer.cs

public sealed class Customer : IEntity, IAggregateRoot
{
    public Guid Id { get; private set; }
    public Name Name { get; private set; }
    public SSN SSN { get; private set; }
    public IReadOnlyCollection<Guid> Accounts
    {
        get
        {
            IReadOnlyCollection<Guid> readOnly = _accounts.GetAccountIds();
            return readOnly;
        }
    }

    private AccountCollection _accounts;       

    public Customer(SSN ssn, Name name)
    {
        Id = Guid.NewGuid();
        SSN = ssn;
        Name = name;
        _accounts = new AccountCollection();
    }

    public void Register(Guid accountId)
    {
        _accounts.Add(accountId);
    }

    private Customer() { }

    public static Customer Load(Guid id, Name name, SSN ssn, AccountCollection accounts)
    {
        Customer customer = new Customer();
        customer.Id = id;
        customer.Name = name;
        customer.SSN = ssn;
        customer._accounts = accounts;
        return customer;
    }
}