Dependency Injection Breaks Encapsulation

[tony_marston404]

@Jeff_Mott

How does class X get its dependencies? For example, if I were to take one of your classes, Tony, and try to use it in one of my projects, it wouldn’t work, and one of the most fundamental reasons is because your code requires your singleton class to manage dependencies.

If you read my article you will see that I specifically stated that I do not write class libraries where developers can pick and choose which classes they want to use. I write complete applications, and the Person class is part of my “Party” subsystem which you either use in its entirety or do not use at all.

It is not correct to say that my singleton class “manages” any dependencies. It simply returns an instance of the named class and the name of that class is hard-coded in the object which wants to use the dependency.

@Jeff_Mott

Imagine if the classes I wrote used my container, and the classes you wrote used your container, and the classes that everybody else wrote each use their own respective container… then no dependency could ever be shared. None of us would ever be able to share code, and we’d all be forced to keep re-inventing the wheel, because every one of us would be creating wheels that only work on our own personal car.

I repeat, I do not write sharable class libraries, I write complete applications. You either use the whole application or you don’t use it at all.

@Jeff_Mott

At this point, I presume your response would be something along the lines of, “I don’t have that problem. I don’t use other people’s code. I don’t care if they can’t use mine.” And that certainly all seems true. All I can say is, that kind of approach leads to code that only works with itself and is therefore widely considered to be bad. The consensus seems to be that good code plays nice with others.

That is only relevant if my intention is to write code that can be shared outside of my framework. It is not, and I am not going to spend any time in providing a mechanism to do something which I know that I will never do. This would violate the YAGNI principle,

@Jeff_Mott

But don’t make the mistake of thinking that’s considered the best way to do things.

It is also not the “best way to do things” if you implement DI in circumstances where it was not designed to be used. My article contains the following description:

Dependency Injection is the solution to the problem where an object has a dependency which can be supplied from a variety of interchangeable sources of the same type.

In the above example the Address object is not interchangeable with any other object as address data can only be obtained from a single place, so if it is not interchangeable it is not a good candidate for DI.