Representing data as code

1_Before.cs

public class Industry : Entity
{
    public const string CarsIndustry = "Cars";
    public const string PharmacyIndustry = "Pharmacy";
    public const string MediaIndustry = "Media";

    public string Name { get; private set; }
}

public class Customer : Entity
{
    public Industry Industry { get; private set; }
    public EmailCampaign EmailCampaign { get; private set; }

    public Customer(Industry industry)
    {
        UpdateIndustry(industry);
    }

    public void UpdateIndustry(Industry industry)
    {
        EmailCampaign = GetEmailCampaign(industry);
        Industry = industry;
    }

    private EmailCampaign GetEmailCampaign(Industry industry)
    {
        if (industry.Name == Industry.CarsIndustry)
            return EmailCampaign.LatestCarModels;

        if (industry.Name == Industry.PharmacyIndustry)
            return EmailCampaign.PharmacyTips;

        if (industry.Name == Industry.MediaIndustry)
            return EmailCampaign.MediaNews;

        throw new ArgumentException();
    }
}

public enum EmailCampaign
{
    LatestCarModels,
    PharmacyTips,
    MediaNews
}

public class CustomerController
{
    public void CreatePharmacy(string name)
    {
        Industry industry = _industryRepository.GetByName(Industry.PharmacyIndustry);
        var customer = new Customer(industry);
        _customerRepository.Save(customer);
    }
}

2_After.cs

public class Industry : Entity
{
    public static readonly Industry Cars = new Industry(1, "Cars");
    public static readonly Industry Pharmacy = new Industry(2, "Pharmacy");
    public static readonly Industry Media = new Industry(3, "Media");

    public string Name { get; private set; }

    private Industry(int id, string name)
    {
        Id = id;
        Name = name;
    }
}

public class Customer : Entity
{
    public Industry Industry { get; private set; }
    public EmailCampaign EmailCampaign { get; private set; }

    public void UpdateIndustry(Industry industry)
    {
        EmailCampaign = GetEmailCampaign(industry);
        Industry = industry;
    }

    private EmailCampaign GetEmailCampaign(Industry industry)
    {
        if (industry == Industry.Cars)
            return EmailCampaign.LatestCarModels;

        if (industry == Industry.Pharmacy)
            return EmailCampaign.PharmacyTips;

        if (industry == Industry.Media)
            return EmailCampaign.MediaNews;

        throw new ArgumentException();
    }
}

public class CustomerController
{
    public void CreatePharmacy(string name)
    {
        var customer = new Customer(Industry.Pharmacy);
        _customerReposoitory.Save(customer);
    }
}

[Fact]
public void Updating_industry_updates_email_campaign()
{
    var customer = new Customer(Industry.Pharmacy);

    customer.UpdateIndustry(Industry.Cars);

    Assert.Equal(EmailCampaign.PharmacyTips, customer.EmailCampaign);
}

public class IndustryTests
{
    [Fact]
    public void All_requires_values_are_in_DB()
    {
        Verify(1, Industry.Cars);
        Verify(2, Industry.Pharmacy);
        Verify(3, Industry.Media);
    }

    private void Verify(int industryId, Industry hardCodedInstustry)
    {
        using (var unitOfWork = new UnitOfWork())
        {
            var repository = new IndustryRepository(unitOfWork);
            Industry industry = repository.GetById(industryId);

            Assert.Equal(industry.Id, hardCodedInstustry.Id);
            Assert.Equal(industry.Name, hardCodedInstustry.Name);
        }
    }
}