In this chapter, we looked at how the intent of our users can be represented as commands that those users send to our system. We learned how these commands can be handled, looked at several examples of the command handler pattern, and then got to the application service.
We looked at API versioning; although it is not directly related to the book topic, the practice is too important to ignore. We will touch upon the versioning topic in Chapter 10, Event Sourcing.
Our application service grew throughout this chapter, and we used one of the latest features of C#, a gift from the functional world, called advanced pattern matching. We used this feature to simplify the application service interface, which ended up having just a single method. By doing this, we also enabled using a composition, yet another functional-style approach, to chain command handling with operational...