Caution
Without a clear understanding of polymorphism and composition, OCP and the rest of the principles of SOLID won't make much sense...
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we want to filter by Customer types
the product owner wants to have a filter by subscription as well
What is the motivation at (1) to introduce the
ISpecification<T>
concept?There isn't any.
Introducing it at (1) requires prescience that (2) will be required in the future. However time and time again developers have been shown to be terrible at predicting future changes, often increasing complexity to gain generality that never pays off or worse gets in the way of change that is needed later.
The more realistic scenario is that the simple version is implemented at (1) and that
ISpecification<T>
is introduced at (2) - which means that nothing was ever "closed to modification".Kevlin Henney's rant about the Open-Closed Principle.
The idea behind this article is that we should write clean code from the beginning and the scenario i described was just to introduce the problem and show that the first code written was not good enough to follow the design principle.
If we talk about the efficiency of the SOLID principles, we have pros and cons but for me the pros are more than cons.
Thanks for your comment.
The final code only makes sense when both requirements (1, 2) are known.
Introducing
ISpecification<T>
when only (1) is known is a guess. It may be an educated guess but the investment doesn't really pay off until (2) materializes - before that the complexity it introduces is non-essential.Compare that to
It acknowledges that earlier we may have had insufficient information to create the optimal design (to grant the freedom to be changed in the way needed) but now that we know what needs to change, we adapt to reflect our new state of knowledge.
Our version 1 "state of knowledge" simply leads to
And version 2 might simply be
At this point we may simply decide that duplication is preferable to the wrong abstraction and just leave it.
If we do decide to go ahead with
ISpecification<T>
because some followup conversations with the stakeholders strongly suggested that there may be other filter types needed later but some code we don't have access to already usesFilterByType
we may even end up withSo the real reason to be cautious with the Open-Closed Principle is when it requires you to predict the future - because when you predict the future you'll likely be wrong (YAGNI).
"Be careful when choosing the areas of code that need to be extended; applying the Open-Closed Principle EVERYWHERE is wasteful, unnecessary, and can lead to complex, hard to understand code."
Head First Design Patterns; Eric Freeman & Elisabeth Freeman, 2004; p.87
Robert C. Martin wrote about OCP again in 2014 - essentially advocating a Plugin Architecture. When you design a plugin architecture you have already decided up front along which lines your system needs to be extensible (supported hopefully by good evidence of that necessity/benefit). So OCP is presented as a principle for systems design - not class design.
Also note that
ISpecification<T>
has exactly one method. As Scott Wlaschin observes function types take SRP and ISP to the extreme.So given what C# is capable of nowadays
seems like a much less cumbersome way to move forward.
I completely agree with you on YAGNI.
Also i know that applying the Open-Closed Principle EVERYWHERE is wasteful, unnecessary, and can lead to complex, hard to understand code and fully agree with it.
But seriously I’m not a fun of duplicated code and the is a lot of solutions to avoid that !
The idea behind my article was to explain how we can implement the OCP principle inside a software in different ways. This is an article to simplify things to software developers in my point of view.
And in the conclusion a noticed that :
I can’t see what was wrong ?
To conclude :
I think, we both agree that SOLID are principles and not rules!!
Thank you for these comments which have raised this thread and i hope that it will be conclusive debate.
DRY isn't about removing repetition or duplication: "every piece of knowledge must have a single, unambiguous, authoritative representation within a system". Sometimes code can look similar but is unrelated.
(POV: Development by Slogan with DRY: Part 2, The Tower of Coupling)
The setup.
If the article would have started by stating that it was known up front that both
ByType
andBySubscription
filtering would be needed and that other, yet to be determined kinds of filtering may still need to be added in the future, that would have laid out the background of a clear and necessary dimension of extensibility that OCP could apply to.As it is, it was presented as if OCP somehow should make it obvious which dimension of extensibility is needed when only
ByType
filtering is required. Initially there are no actual dimensions of variability indicated. That only happened once theBySubscription
requirement surfaced later.This sets up the unrealistic expectation that
IFilter<T>
should be introduced at the very beginning (which is revisionist) when in fact the value of that abstraction only becomes clear much later.In a realistic scenario you would start with
FilterByType
and end up atApply
- which means that the code isn't "closed to modification" as you are "opening it for extension".In my example i apply OCP just in time and this what should we do and i Refactor to Open-Closed.
YAGNI
The Open Closed Principle - The Clean Code Blog by Robert C. Martin (Uncle Bob)
Protected Variation:The Importance of Being Closed
[2]Hall, 1988. [2] Refactoring, Martin Fowler, Addison Wesley, 1999
The Open Closed Principle - The Clean Code Blog by Robert C. Martin (Uncle Bob)
In C# the extension method is used as a solution to OCP. Over the years we've learned the principles of the Repository pattern which is, in essence to make all collections generic. This satisfies the ability to inject any type into a collection. Finally Language Integrated Query ability gives us precise filtering.
The open/closed principle is a guideline for the overall design of classes and interfaces and how developers can build code that allows change over time. By ensuring that your code is open to extension but closed to modification, you disallow future changes to existing classes and assemblies, which forces programmers to create new classes that can plug into the extension points. That’s the idea ;)
Yes, and my point is C# has a built-in construct that automatically creates extensions. It is very cool.
What would be the difference to an implementation with the C# LINQ type
Predicate<T>
except the strong typing of your filters?I'm thinking of an implementation like this:
My thoughts on Predicate Method:
My final thought logic is that if you are working on a complex business application where business requirements are known up front but may change over time, then i would use Specification pattern.
The use of a design pattern It gives you highest modularity and flexibility and of course testability