Part 1 provided an overview of DI, discussing the purpose and benefits of DI. Even though chapter 3 contained an extensive example, we’re sure the first chapters still left you with some unresolved questions. In part 2, we’ll dig a little deeper to answer some of those questions.

As the title implies, part 2 presents a complete catalog of patterns, anti-patterns, and code smells. Some people dislike design patterns, because they find them dry or too abstract. Personally, we love patterns, because they provide us with a high-level language that makes us more efficient and concise when we discuss software design. It’s our intent to use this catalog to provide a pattern language for DI. Although a pattern description must contain some generalizations, we’ve made each pattern concrete, using examples. You can read all three chapters in sequence, but each item in the catalog is also written so that you can read it by itself.

Chapter 4 contains a mini catalog of DI design patterns. In a sense, these patterns constitute prescriptive guidance on how to implement DI, but you should be aware that we don’t consider them to be of equal importance. Constructor Injection and Composition Root are by far the most important design patterns, whereas all the other patterns should be treated as fringe cases that can be applied in specialized circumstances.

Whereas chapter 4 gives you a set of generalized solutions, chapter 5 contains a catalog of situations to avoid. These anti-patterns describe common, but incorrect ways to address typical DI challenges. In each case, the anti-pattern describes how to identify occurrences and how to resolve the issue. It’s important to know and understand these anti-patterns to avoid the traps that they represent, and, just as chapter 4 presents two dominatingly important patterns, the most important anti-pattern is Service Locator, the antithesis of DI.

As you apply DI to real-life programming tasks, you’ll run into some challenges. We think we’ve all had moments of doubt where we feel that we understand a tool or technique, and yet we think, “In theory, this may work, but my case is special.” When we find ourself thinking like this, it’s clear to us that we have more to learn.

During our career, we’ve seen a particular set of problems appear again and again. Each of these problems has a general solution you can apply to move your code towards one of the DI patterns from chapter 4. Chapter 6 contains a catalog of these common problems, or code smells, and their corresponding solutions.

We expect this to be the most useful part of the book, because it’s the most enduring. Hopefully, you’ll return to these chapters months and even years after you first read them.