What is Covariance and Contravariance in C#: A Complete Overview

Are you a programmer struggling to understand what are the covariance and contravariance in C#? Look no further! In this video, I'm going to cover everything you need to know about covariance and contravariance in C#. As an expert in C# programming, I know that understanding covariance and contravariance is essential for working with generics and functional programming. That's why we've created this comprehensive video to explain the differences between covariance and contravariance, and how they can be used in programming. My video covers topics like the object substitution principle, variance, contravariant, covariance in C#, and contravariance. I'll walk you through examples and provide a clear explanation of how these concepts work in practice.⚡️❤️

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⌚ 0:00 Today's Topic: Explained covariance and contravariance in c#
⌚ 0:22 What is the fundamental principle of object-oriented programming?
⌚ 1:16 How Object Substitution Principle works on regular types
⌚ 3:36 What is Variance?
⌚ 4:46 What is contravariant?
⌚ 5:12 What is the consequence of having this producer as covariant and this consumer as contravariant interfaces?
⌚ 8:48 What would happen if we tried to assign these interface references one to another?
⌚ 11:30 Courses and Outro
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#covariance #contravariance #generics #variant #variance #csharp #dotnet #zoranhorvat
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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 YouTube channel, you can find shorter video forms focused on clarifying practical issues in coding, design, and architecture of .NET applications.❤️
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[TrungTran-rztz]

I felt off my chair when he said "it is very simple indeed" at the end

[phyberoptyk]

You sir never cease to blow my mind with your incredible insight. Thank you!

[dracuul]

Thanks! This has to be the best explanation of the topic I've come across, with a nice exhaustive set of examples.

[gorotim]

Деда, ты лучший. Я понял разницу между этими принципами, благодаря тебе.

[davidpccode]

This explanation is not only perfect, it is a work of art.. Thanks!

[dererzherzog]

A great and very systematic explanation of the concepts of Covariance and Contravariance and their practical implications in various contexts (reference assignment, method parameters and interfaces).

[nightwheel_c]

Thank you! very clear, and I love your talking speed, it makes me easy to follow

[learner]

Zoran sounds like a very intelligent person. Thanks for the video. Never x my mind to look at Base and Derived this way !

[piotrrybka]

Very good explanation, but not because of the examples, but for the initial introduction of the cause-effect origins of both terms. I think this is the actual eye-opener which explains the essence of both concepts.

I would only emphasize as strongly as I can that co- and contravariance are both very general concepts, and it's not about C# interfaces, in/out keywords, parameters and return values. The examples you gave may have been switched or given in another language, but the general idea is the following:

If our new type N that we are building (the interface in your example), follows the order of derivation strictly, then it's COvariant, otherwise, it's CONTRAviariant.

To put it more clearly:

if there is type B derived from A, so B:A

and I have a generic type N<T>, then

if
for N<A> I can get (in whatever way) only A
in other words for shrinking⬇ of N, the output is also shrunk⬇
and
for N<B> I can get both A and B
in other words, for expanding⬆ of N, the output is also expanded⬆
then N is COvariant

if
for N<A> I can get both A and B
in other words for shrinking⬇ of N, the output is expanded⬆
and
for N<B> I can get only B
in other words for expanding⬆ of N, the output is shrunk⬇
then N is CONTRAvariant.

But of course, it's still nice to see an implementation, which although not fully developed (you're assigning null! to your interfaces and try to use them), could easily be used in a compilable code.

[tkjyxrb]

The terms may be actually coined from math, there are covariant and contravariant functors in category theory and they very nicely demonstrate whan happens in C#. A covariant functor f (or just "functor") has a mapping `(a ~> b) -> (f a ~> f b)`, a contravariant functor g has a mapping `(a ~> b) -> (g b ~> g a)`. Now, imagine that ~> stands for "is subtype of", `f` and `g` are your types with variance definition on their type parameters, and you get variance relations in OOP: from `A : B` follows `F<A> : F<B>` if `F<out T>`, from `A : B` follows `F<B> : F<A>` if `F<in T>`. Now, about those `in` and `out`: functions are functors, covariant in their output and contravariant in their input.

[Rhysling]

Excellent explanation. I’ve heard several and this is the best. I may actually understand cov/ contra now.

[muhammedalikhan]

Amazingly simple explanation. Thank you so much for this!

What advantage gives using "MyInt<out T>" versus "MyInt<T>" not specifying anything? Because if it doesn't change the behavior then it can be just ommited always, right?

[alecmatias]

Thanks! This explains it better than other tutorials.

[brenodev]

Best explanation ever. You're the man! Thanks Zoran :)

[mumk]

you made it so so so so so clear on this topic, thank you sir!

[imaginative-monkey]

Yeah, it was so easy, but for some reason, it was the first time I could understand it! 😆 👍

[somayehsheikhi]

Wow Thanks a lot Zoran, Finally I could understand these concepts. you are amazing .

[Darebitk]

¡Gracias por compartir!, los ejemplos estuvieron muy claros y despejan toda duda acerca de estos dos conceptos

[NoDoubt]

Great job! That was the best demo on this topic I've found!