Breaking Bad: Liskov’s Substitution Principle (LSP) violation

Recently I was watching a very nice explanation of LSP on Christopher Okhravi’s YouTube channel. After approximately five minutes of watching this video I wanted to know one thing: how can I actually break it?

Because, you know, breaking things when you learn gives you better understanding of how they work. Am I right?

Let’s start with a simple Adder class and a class which will inherit from it:

The AdvancedAdder class does the same that Adder class does for now. It adds two numbers.

Now we need a UnitTest that will check if the Liskov’s Substitution Principle was violated. It’s done by next three classes:

We are ready to break it!

Well to be honest I spent a half of an hour trying to figure it out.  

Adding third parameter to make the AdvancedAdder sum three numbers didn’t break LSP. It just added another Add method with a different signature extending the class functionality.

Changing parameters type did not break it either. Again it just added one more method to the class.

Then I came up with an idea that I HAVE to override Add method to make it to do something absolutely not related to the Adder class. Subtracting for example:

I did it! 


It’s not that simple to violate Liskov’s Substitute Principle without overriding a method. Can you do it without “override”?

P.S. When I wrote this post two years ago I was confused by LSP. You can probably see it in the code above.

After some consideration, I decided to keep this post as is. Nostalgy, you know 🙂

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