Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
212 lines (156 sloc) 6.13 KB
layout author title subTitle date bigimg img categories tags interesting toc
post
Wouter Van Schandevijl
DateStart + DateEnd
And Their Evil Relatives
2019-06-19
url desc origin
valueobjects-big.png
Photo by freestocks.org
url desc origin
valueobjects.jpg
Photo by Seth Macey
design
war-story
url desc
Patterns of Enterprise Application Architecture - Martin Fowler
title
🙈 See No Evil

ValueObjects

{: .hide-from-excerpt}

A ramble against objects containing DateStart & DateEnd pairs of properties.

Some of their equally evil relatives:

  • DateStart & TimeSpan or Day/Month only in DateTime, etc.
  • decimal Money & string Currency
  • decimal X/Y and/or Width/Height

This is the case for the ValueObject:

A small simple object, like money or a date
range, whose equality isn't based on identity.
— Martin Fowler

Basic Example

Consider the following class:

class Person {
    public string FirstName;
    public string LastName;
    public string Initials;
    public string Address;
    public string Street;
    public int HouseNumber;
    public string Telephone;
    public string Mobile;
    public string Email;
    public string WorkAddress;
    public string WorkStreet;
    public string WorkHouseNumber;
    public DateTime ActiveStart;
    public DateTime? ActiveEnd;
    // ... and many more?
}

And how beautiful it could've been:

class Person {
    // The first version was quite obfuscated, while in this version it's
    // quite obvious what's in it by merely glancing at the declaration.
    public Name Name;
    public Contact Contact;
    public Address HomeAddress;
    public Address WorkAddress;
    public DateRange Active;
}

class Name {
    // Please don't repeat the Name for each property here
    // Considering the current class, we know the context!
    public string First;
    public string Last;
    public string Initials;
}

Advantages

For the IDE

Instead of a long list of properties that may be difficult to navigate in your IDE, where it is difficult to spot that one property you need, one could instead split it up in things that logically belong together to keep intellisense small and focused.
Keep data that belongs together - together!

For the UI

Such stuff is usually displayed together. All the way down to the frontend, passing an Address to an AddressComponent is very preferable over passing those properties in separately.

For Expressiveness

Without our precious ValueObject, we would end up creating all sorts of helper classes for often sought after functionality, all scattered around the codebase, some perhaps getting duplicated by different programmers or even for different UserStories.

Some of these helper methods would be much better off being members of a ValueObject instead. We get better IDE discoverability and rather than having to pass some or all properties of an entity into an algorithm, we invoke a well-named method on the ValueObject in true TellDontAsk style.

And isn't that what Object Oriented Design is all about? 😇

Recognition

If you look, ValueObject opportunities will present themselves.

Technical ValueObjects

Classes are very technical in nature and so there'll be purely technical candidates like, for example, a (still?) popular one:
CreatedOn + CreatedBy and UpdatedOn + UpdatedBy.
Why clutter (all) your entities with 4 superfluous properties if you can hide them behind a single Audit ValueObject?

Domain ValueObjects

It is the ValueObjects derived from the Business Domain that often offer the most value however. Helper functions for a DateRange only go as long as a IsIn(DateTime). For Domain ValueObjects, this can be whatever rabbit Marketing pulls out of their hat depending on the current lunar phase, so the possibilities are endless!

Not Invented Here Syndrome

I never thought I'd ever say this: Do not use a package. Yes, there is that well-known and fully-fledged DateRange package out there. 80% of its features are probably not useful for your problem domain though. Again you get a long list of methods that basically clutter your IDE. Super handy features that are applicable only in your domain will most likely not be available, as your business probably has very particular requirements.

Business Domain Example

// A domain where we work with measured stuff
class MeasurementResults {
    public decimal Value1;
    public int Value1Precision;
    public decimal Value2;
    public int Value2Precision;
    public decimal Value3;
    public int Value3Precision;
}

// Could be turned into
class MeasurementResults {
    public Measurement Result1;
    public Measurement Result2;
    public Measurement Result3;
}

class Measurement {
    public decimal Value;
    public int Precision;
}

Random Thoughts

The True Enemy?

Well I guess that missing ValueObjects are often an unfortunate side-effect of using a relational database. Your SQL database simply likes things flat. Your ORM probably has something like Custom Types (Hibernate), Complex Types (EF) or Owned Entities (EF Core). If you use an ORM it can probably do the ValueObject creation for you.

Derive, Don't Store

If you can calculate something from existing state, do so. Once stored separately, it will, at some point be out of sync, I guarantee it.

class Person {
    public string FirstName;
    public string LastName;

    // This simply does not get out of sync
    // Otherwise each day is trying to remember if the
    // FullName starts or ends with the FirstName
    // +You have less state to manage, so hurray!
    public string FullName => $"{FirstName} {LastName}";
}

Conclusions

Not creating a value object for coherent pairs of value types is a missed opportunity? 😂

You can’t perform that action at this time.