It sounds like the title of a Big Bang Theory episode, but its not, instead is an all to common problem that breaks the single responsibility rule that I see regularly.
And I’ve seen first hand how much butt hurt this can cause, a friend of mine (Tomz) spent 6 weeks trying to update a library in a website, the library having a breaking change.
The root cause of this issue comes from the following scenario, My Project depends on my Library 1 and 2. My Libraries both depend on the same 3rd party library (or it could be an internal one too).
Now lets take the example that each library refers to a different version.
Now which version do we use, you can solve this issue in dotnet with assembly binding redirects, and nuget even does this for you (other languages have similar approaches too). However, when there is a major version bump and breaking changes it doesn’t work.
If you take the example as well of having multiple libraries (In my friend tomz case there was about 30 libraries that depended on the logging library that had a major version bump) this can get real messy real fast, and a logging library is usually the most common scenario. So lets use this as the example.
So what can we change to handle this better?
I point you to the single responsibility principle. In the above scenario, MyLibrary1 and 2 are now technically responsible for logging because they have a direct dependency on the logging library, this is where the problem lies. They should only be responsible for “My thing 1” and “My thing 2”, and leave the logging to another library.
There is two approaches to solve this that i can recommend, each with their own flaws.
The first is exposing an interface that you can implement in MyProject,
This also helps with if you want to reuse you library in other projects, you wont be dictating to the project how to log.
The problem with this approach though is that you end up implementing a lot of ILoggers in the parent
The other approach is to use a shared common Interfaces library.
The problem with this approach however is when you need to update the Common interfaces library with a breaking change it becomes near impossible, because you end up with most of your company depending on a single library. So I prefer the former approach.