gistfile1.cs

    public class Unregistered : ITennantRegistration
    {
        public ITennantRegistration Regsister()
        {
            return new Registered();
        }
    }

    public class Registered : ITennantRegistration
    {
    }

    public class Tennant
    {
        protected ITennantRegistration registration;
        public Tennant()
        {
            this.registration = new Unregistered();
        }

        public void Register()
        {
            var reg = registration as Unregistered;
            if(reg!=null)
                registration = new Registered();
        }

        public void AddUserAccount(string name, string password)
        {
            var reg = registration as Registered;
            if (reg != null)
                this.User = new User(reg, name, password);
        }

        bool canRegister()
        {
            return true;// check invariant here
        }
    }