Does the primitive obsession anti-pattern apply to collections? In other words, should you introduce a custom class for a collection? 1. Preamble Primitive Obsession is an anti-pattern that takes place when you overuse primitive types, especially to model your domain. This anti-pattern is widely known in both DDD and functional programming communities…

Today, we’ll talk about an important question: how does the application database fit into the concept of Always-Valid Domain Model? In other words, is the database part of the always-valid boundary or should you consider it an external system and validate all data coming from it? 1. Always-Valid Domain Model Just a quick reminder…

There’s an interesting controversy between two DDD topics: the Specification pattern and the Always-Valid domain model. 1. Specification pattern I’ve written multiple articles about the Specification pattern, including another controversy: Specification vs CQRS patterns. This time, we’ll look at the contradiction the Specification pattern has with another DDD concept: Always-Valid Domain model. Just…

Are static methods good or bad? Over the course of my career I did a full circle on this topic. In this article, I’ll try to describe this evolution and the reasoning behind it. 1. Oh, cool, static methods! After learning about static methods for the first time, most people (myself included) become enthusiastic. …

I was re-reading some old articles about always-valid domain models, including this one form Jeffrey Palermo and the response to it from Greg Young. I highly recommend that you read them both if you haven’t already. This post is another response to Jeffrey’s article, which will hopefully complement Greg’s one. 1. Not-always-valid domain model …

I’m continuing the topic of domain model purity. This time, we’ll look at it with regards to getting the current date and time. By the way, be sure to subscribe to my email list. Not all discussions fit the format of a blog post (including some shorter takes on the…

I’ve been meaning to write this article for a long time and, finally, here it is: the topic of domain model purity versus domain model completeness. Domain model completeness In this article, we’ll talk about a trilemma that comes up in each and every project. To best describe this trilemma, we need to…

How to unit test an abstract class? Or a whole class hierarchy that depends on that abstract class? Let’s see. Unit testing an abstract class Imagine you work on a people software in a university and have the following code: public class Student { public string Name { get; set; } …

In this post, we’ll make a deep dive into the DRY and DAMP principles and will talk about the false dichotomy around them. The DRY and DAMP principles The DRY principle stands for “Don’t Repeat Yourself” and requires that any piece of domain knowledge has a single representation in your code base. In other words…