Unveil the Mystery of the Liskov Substitution Principle
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Have you ever exclaimed that this or that method violates LSP, looking so bright in the team but, deep inside, hoping nobody would ask you to explain?
Have you ever raised your voice when someone mentioned LSP but, deep inside, you knew it was because you lacked the words to say it better?
Spare a few minutes to learn what makes LSP so hard, but also to learn how to sort it out in your head once and for all.
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👨 About Me 👨
Hi, I’m Zoran, I have more than 20 years of experience as a software developer, architect, team lead, and more. I have been programming in C# since its inception in the early 2000s. Since 2017 I have started publishing professional video courses at Pluralsight and Udemy and by this point, there are over 100 hours of the highest-quality videos you can watch on those platforms. On my CZcams channel, you can find shorter video forms focused on clarifying practical issues in coding, design, and architecture of .NET applications.❤️
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I love that channel, thanks for the contribution
Thinking Why: it's a threshold for mid level programmer. Too often I see programmers who do "by example" without any reasoning Why.
LSP is probably more obvious/intuitive than you think; it just means more lenient inputs and more restrictive outputs are legal. Imagine something like `Object doStuff(ICollection input)` (i'm a java dev, don't get hung up on api specifics!).
If you subclass this, it's completely fine to return, well, anything because it's almost impossible to break the contract of `Object` (its also not very useful!). Any callers expecting `Object` as a return from the super class will be just fine w/you returning, say, a string.
However, you can't make the subclass take something like an `IList` because the caller wasn't expecting that from the parent. Note that if the parent had required an `IList` its perfectly fine for you to loosen the contract and take an `ICollection` (this is usually fairly intuitive)
It can be a bit trickier w/o explicit types. Imagine a function that returns an int, but semantically its always positive. You could subclass that and now return negative values as well, but that breaks the contract. w/o some kind of `PositiveInteger` type, you can't express this constraint as easily.
Yes that is pretty much what I said in the video.
@@zoran-horvat I find concrete examples drive things home. ymmv.
@@zoran-horvatYeah but this time Adam explains it with the code :) It is quite different from your side that you don't use code for the concepts. But lovely to see your explanation. Both of you explains the concepts beautifully. Thanks.
The slow mode of your speech and clear pronunciation, which are present, are the advantage of your content.
Teaching methods matter.
Hi Zoran,
I've been following your work for a while and I'm curious about your audience. Do you find you attract mostly developers who are new to the field(2-5 years of experience), or those with more experience like myself (15+ years)?
From analytics, most of the audience are senior programmers.
Yeah ! Science!
I like this format!
@zoran-horvat I would love to see your approach to unit testing. E.g. do you follow TDD. What sort of rational do you use on when to right tests and how would that fit into you approach on "Iterative development" (busy with your course on Udemy). I sense many similarities in this gradual progression and emergent design as with red green refactor.
That is an interesting topic.
can you please make a video how to be clear with Oops Concept from our mind and what we have to do to be crystal clear with Oops Concepts.
@zoran-horvat, would you please make a video to demystify the Law of Demeter? That'd be a bang! Thank you.
I would have to start with "law of Demeter is not a law". Are you still in?
Yep, it's a rule, not a law per se
@@aflous why? who said that? do you know the cost of violating it?
@@TopKoder Actually, I don't know the cost of violating it... Think carefully about your next line.
@zoran-horvat sorry, I didn't get the sentiment of the last sentence. Is it humorously intended?
With regards to the cost of violating LoD. I believe it's increased blast radius (= regression risk, = cost of rework) of any change.
The incredible!
Thanks for the explanation. I guess I'm quite a slow person, so I'm not able to properly explain it like you do. My thought process whenever facing a question abt LSP of a case usually likes this: "Ok... recall the Rectangle and Square example... now, does this case have anything similar to that example? Yes, no?"
Oh, the square and rectangle example is a corner case that spans corners of several programming theories. I would stay away from it when only speaking of LSP.
@@zoran-horvat yeah... with that I can only answer "No" and "Maybe" whenever someone asks me if a case is breaking LSP.
@@petervo224 Actually, you can do more. You can ask for the definition of the interface in terms of the abstract properties it must satisfy and then ask in which way the implementation breaks them.
It is possible that the implementation is breaking some attributes or not breaking any of them. That is the difference between violating the LSP and not violating it, quite objectively.
Any other "violation" that causes disagreement might be viewed as a nonfunctional requirement, but not as LSP.
Still had to read around what class invariants meant. Not being formally trained, still struggle with terminology. I've got the same problem with natural languages, education in UK sucks.
Otherwise, got you. Is clear.
Next step, how do I test for adherence to LSP?
That is a good question. There is a strong correlation between postconditions and unit tests.
@@zoran-horvat Video coming up? Some code examples? Speaks clearer than abstract terms.
@@nickbarton3191 Yes, the video about LSP done in code is currently in the process and should come out soon.
@@zoran-horvatSo good, found an oracle that makes practical sense.
I like the beard, very calm and well aged look.
I'm young.
@zoran-horvat I'm 22 and look better with a light beard. Well aged doesn't mean old
The override method will always have different implementation, so what's the point of override in the first place😅 Then like you said, if have same signature and same responses it's not breaking anything 👍🏻
Not exactly the same responses. It is a detective game, or the game lawmakers play: Your role as a programmer is to define what's legal and what's not. Everything that is legal must be acceptable by tests, full stop.
Programmers are often picky, and what they actually end up doing is micromanage their classes.
A class is free to do (i.e. implement) whatever it finds fit unless that is in the illegal part for the thing it models.
Once you get to viewing the world through that pair of glasses, you will never want to look back.
@@zoran-horvat👍🏻👍🏻
The problem with SOLID is that if you have 10 developers in a room, one can tell you all about it, and everyone will agree. If you instead take all the 10 developers in separate rooms and ask them individually, you will have more than 10 explanations about the SOLID principals. People who disagrees with this, is most likely the one person in the first sentence.
That may be true in teams where principles are taken religiously. I don't approve of such views on SOLID nor any other set of principles.