This is a post that I’ve had in my drafts folder for nearly two years. Well, I should say I’ve had the title and a few haphazard notes in my drafts folder for nearly two years; I’m writing the actual post right now. The reason I’ve had it sitting around for so long is twofold: (1) it’s sort of a subjective, tricky topic and (2) I’ve struggled to find a concrete stand to take. But, I think it’s relatively important, so I’ve always vowed to circle back to it and, well, here we are.
The draft was first conceived when I’d given a presentation about testability and inversion of control — specifically, using dependency injection via constructors and setters to achieve these ends. I talked, among other things about the Open/Closed Principle, and how this allows modifications to system behavior that favor adding over editing code. The idea here is that we can achieve new functionality with a minimum of violence to the code base and in a way for which it is easy to write unit tests. Everyone wins, right?
Well, not everyone was drinking the Kool-Aid. I fielded a question about encapsulation that, at the time, I hadn’t prepared to answer. “Doesn’t this completely violate encapsulation?” I was a little poleaxed, and sputtered out an answer off the cuff, saying basically, “well, not completely….” I mean, if you’ve written a class that takes ILogger in its constructor and uses it for logging, I control the logger implementation but you control when and how it is used. So, you encapsulate the logger’s usage but not its implementation and this stands in contrast to what would happen if you instantiated your own logger or implemented it yourself — you would encapsulate everything and nothing would be up to me as a client of your code. Certainly, you have more encapsulation. So, I finished: “…not completely…. but who cares?!?” And that was the end of the discussion since we were out of time anyway.
I was never really satisfied with that answer but, as Creedence Clearwater says, “time and tears went by, and I collected dust.” When I thought back to that conversation, I would think to myself that encapsulation was passe in the same way that deep inheritance hierarchies were passe. I mean, sure, I learned that encapsulation was one of the four cornerstone principles of OOP, but so is inheritance, and that’s kind of going away with “favor composition over inheritance.” So, hey, “favor dependency injection over encapsulation.” Right? Still, I didn’t find this entirely satisfying — just good enough not to really occupy much of a place in my mind.
But then I remember a bit of a brouhaha last year over a Stack Overflow question. The question itself wasn’t especially remarkable (and was relatively quickly closed), but compiler author and programming legend Eric Lippert dropped by to say “DI is basically a bad idea.” To elaborate, he said:
There is no killer argument for DI because DI is basically a bad idea. The idea of DI is that you take what ought to be implementation details of a class and then allow the user of the class to determine those implementation details. This means that the author of the class no longer has control over the correctness or performance or reliability of the class; that control is put into the hands of the caller, who does not know enough about the internal implementation details of the class to make a good choice.
I was floored. Here we have one of the key authors of the C# compiler saying that the “D” in the SOLID principles was a “bad idea.” I would have dismissed it as blasphemy if (1) I were the sort to adopt approaches based on dogma and (2) he hadn’t helped author at least 4 more compilers than I have. And, while I didn’t suddenly rip the IoC containers out of my projects and instantiate everything inside constructors, I did revisit this topic in terms of my thoughts.
Maybe encapsulation, in the information hiding sense, isn’t so passe. And maybe DI isn’t a magic bullet. But why not? What’s wrong with the author of a class ceding control over some aspects of its behavior by allowing collaboration? And, isn’t any method parameter technically a form of DI, if we’re going to be pedantic about it?
The more I thought about it, the more I started to see competing and interesting use cases. Or, I should say, the more I started to think what class authors are telling their collaborators by using each of these techniques:
Encapsulation: “Don’t worry — I got this.”
Dependency Injection: “Don’t worry — if this doesn’t work, you can always change it.”
So, let’s say that you’re writing a Mars Rover or maybe a compiler or something. The attitude that you’re going to bring to that project is one in which correctness, performance and reliability are all incredibly important because you have to get it right and there’s little room for error. As such, you’re likely going to adopt implementation preference of “I’m going to make absolutely sure that nothing can go wrong with my code.”
But let’s say you’re writing a line of business app for Initrode Inc and the main project stakeholder is fickle, scatterbrained, and indecisive. Then you’re going to have an attitude in which ease and rapidity of system changes is incredibly important because you have to change it fast. As such, you’re likely to adopt an implementation preference of “I’m going to make absolutely sure that changing this without blowing everything up is easy.”
There’s bound to be somewhat of an inverse relationship between flexibility and correctness. As a classic example, a common criticism of Apple’s “walled garden” approach was that it was so rigid, while a common praise of the same was how well it worked. So I guess my take-away from this is that Dependency Injection and, more broadly, Inversion of Control, is not automatically desirable, but I also don’t think I can get to Eric’s take that it’s “basically a bad idea,” either. It’s simply an exchange of “more likely to be correct now” for “more likely to be correct later.” And in the generally agile world in which I live and with the kind of applications that I write, “later” tends to give more value.
Uncle Bob Martin once said, I believe in his Clean Coders video series, that (paraphrased) the second most important characteristic of good software is that it meet the customer requirements. The most important characteristic is that it be easy to change. Reason being, if a system is correct today but rigid, it will be wrong tomorrow when the customer wants changes. If the system is wrong today but flexible, it’s easy to make it right tomorrow. It may not be perfect, but I like DI because I need to be right tomorrow.