public class UserController
    {
        private readonly Database _database;
        private readonly IMessageBus _messageBus;

        public UserController(Database database, IMessageBus messageBus)
        {
            _database = database;
            _messageBus = messageBus;
        }

        public string ChangeEmail(int userId, string newEmail)
        {
            object[] userData = _database.GetUserById(userId);
            User user = UserFactory.Create(userData);

            string error = user.CanChangeEmail();
            if (error != null)
                return error;

            object[] companyData = _database.GetCompany();
            Company company = CompanyFactory.Create(companyData);

            user.ChangeEmail(newEmail, company);

            _database.SaveCompany(company);
            _database.SaveUser(user);
            foreach (EmailChangedEvent ev in user.EmailChangedEvents)
            {
                _messageBus.SendEmailChangedMessage(ev.UserId, ev.NewEmail);
            }

            return "OK";
        }
    }

    public interface IMessageBus
    {
        void SendEmailChangedMessage(int userId, string newEmail);
    }

    public class MessageBus : IMessageBus
    {
        private IBus _bus;

        public void SendEmailChangedMessage(int userId, string newEmail)
        {
            _bus.Send($"Subject: USER; Type: EMAIL CHANGED; Id: {userId}; NewEmail: {newEmail}");
        }
    }

namespace Book.Chapter8.Circular
{
    public class CheckOutService
    {
        public void CheckOut(int orderId)
        {
            var service = new ReportGenerationService();
            service.GenerateReport(orderId, this);

            /* other work */
        }
    }

    public class ReportGenerationService
    {
        public void GenerateReport(
            int orderId,
            CheckOutService checkOutService)
        {
            /* レポートの生成が完了したらCheckOutServiceを呼び出す */
            /* そんで次の処理に移ってもらう？ */
        }
    }
}

namespace Book.Chapter8.NonCircular
{
    public class CheckOutService
    {
        public void CheckOut(int orderId)
        {
            var service = new ReportGenerationService();
            Report report = service.GenerateReport(orderId);

            /* other work */
        }
    }

    public class ReportGenerationService
    {
        public Report GenerateReport(int orderId)
        {
            /* ... */

            return null;
        }
    }

    public class Report
    {
    }
}

        public void ChangeEmail(string newEmail, Company company)
        {
            _logger.Info(    // ←診断ログ
                $"Changing email for user {UserId} to {newEmail}");

            Precondition.Requires(CanChangeEmail() == null);

            if (Email == newEmail)
                return;

            UserType newType = company.IsEmailCorporate(newEmail)
                ? UserType.Employee
                : UserType.Customer;

            if (Type != newType)
            {
                int delta = newType == UserType.Employee ? 1 : -1;
                company.ChangeNumberOfEmployees(delta);
                // サポートログ
                _domainLogger.UserTypeHasChanged(UserId, Type, newType));
            }

            Email = newEmail;
            Type = newType;
            EmailChangedEvents.Add(new EmailChangedEvent(UserId, newEmail));

            _logger.Info($"Email is changed for user {UserId}");
        }

    public class DomainLogger : IDomainLogger
    {
        private readonly ILogger _logger;

        public DomainLogger(ILogger logger)
        {
            _logger = logger;
        }

        public void UserTypeHasChanged(
            int userId, UserType oldType, UserType newType)
        {
            _logger.Info(
                $"User {userId} changed type " +
                $"from {oldType} to {newType}");
        }
    }

    public class User
    {
        private static readonly ILogger _logger = LogManager.GetLogger(typeof(User));

        public void ChangeEmail(string newEmail, Company company)
        {
            _logger.Info($"Changing email for user {UserId} to {newEmamil}");

            // ather code

            _logger.Info($"Email is changed for user {UserId}");
        }
    }

        public void ChangeEmail(
            string newEmail, 
            Company company,
            ILogger logger
            )
        {
            logger.Info($"Changing email for user {UserId} to {newEmamil}");

            // ather code

            logger.Info($"Email is changed for user {UserId}");
        }
    }