How to create powerful validation with FluentValidation in .NET apps

Validation is a vital part of any application, even if a lot of teams tend to not pay a close attention to this topic. Validation itself can be very hard. Fortunately, the FluentValidation library allows us to create powerful validation rules with ease. In this video we gest started with the FluentValidation library, but we also dive into best practices and practical validators.
#DotNet #DotNetCore #FluentValidation
Chapters
1. Intro: 00:00
2. Real app overview: 01:17
3. Implementing a first validator: 05:09
4. Refactoring to avoid DRY: 16:55
5. Collection validation with conditions: 23:02

[@talkathiriify]

Excellent and smooth explanation. Thank you very much.

[@FreaksSpeaks]

Thanks very much. Love to see more on this topic including how it is wired up to front end as well.

[@Codewrinkles]

That's the point: validators in the domain entities should not be wired to anything :) I have already started to work with some ideas to shape up a video with validation best practices.

[@danielegiovanetti9258]

Very powerful video, well explained. My best compliment.

[@Codewrinkles]

Wow, thanks

[@VinuP2023]

Thank you for making more videos

[@Codewrinkles]

Glad you like them!

[@saefulrahman3478]

thanks, well explained

[@Codewrinkles]

Glad it was helpful!

[@RockRockAndDogs]

Awesome video.

[@Codewrinkles]

Glad you enjoyed it

[@ankitasthana2465]

Nice Video!!!

[@Codewrinkles]

Thanks for the visit

[@mohamedal-qadeery6530]

hello im still a beginner .. i dont understand why i should validate domain model when im going to use FluentValidation to validate the Dtos and if the validate success then im going to map it to domain models using automapper .. so am i wrong that there is no need to validate domain models?

[@talkathiriify]

I just notes that in the ProductValidator constructor you set a rule for product price which must be > 0 line 15 and then in the ValidatePrduct method you check for price

[@Codewrinkles]

Good point!

[@ddieppa]

Thanks very much for this great videos. I like my validators out of the domain layer, I dont like to make the domain dependant on an external package. But then what is the best way to do it? Maybe that could be another video. Thanks again for the great content

[@Codewrinkles]

I think we kind of disagree here. The domain surely needs domain specific validation. in DDD terms this is called controlling invariants. Sure, you can implement your own validation engine but I don't think this justifies the effort. I think we tend to misunderstand the idea that the domain should not depend on other libraries. If your domain works with dates and times, then you should surely use NodaTime for instance. For validation you can use FluentValidation. Also, as I showed in this video, my goal is that my domain entity doesn't depend on the FluentValidation library. And I also showed how to resolve that. My entity only knows that there is a validator that it can use. In that validator you can then change the library if you want or do whatever. It won't break the domain entity.

[@mohamedal-qadeery6530]

Thank you for the video .. can you make a video about crud with nested resources wired up to front end ?

[@Codewrinkles]

There alrready is a video about nested resources in the Asp.Net Core for beginners playlist.

[@ConradAkunga]

I don't believe you need the condition for the ProductImageValidator - if the collection of ProductImages is empty, nothing will be executed anyway.

[@vladmartian]

It would be intresting to me to see how you get the Rich Domain Objects from a Database eventually using EF. Should I use DTOS ?

[@Codewrinkles]

From a database perspective there isn't a big problem if the domain objects are rich. Using validation you make sure that you can only persist valid entities in the db. Then, when you get them, even if EF populates the properties directly through reflection you still have a guarantee that they are valid. But in case of very complicated scenarios going back to the good old data access objects might be an idea. And you map them to the domain entities after retrieval. However, there should really be a reasonable need for this to justify the overhead.

[@vladmartian]

@@Codewrinkles Thanks, for the response. But I have an other question. You create your domain object through a factory, should be the repository injected in the factory? Should I use an other service for creating a new object in a DB?

[@Codewrinkles]

@@vladmartian It's not a full blown factory. Just a factory method. Let's assume we want to create a new product. We'll have a Handler for the CreateProduct command. In the handler we call Product.BuildProduct() and specify all the needed params. The result will be a Product instance on a valida state. I then use the ProductRepository.Add(product) and that's it. You don't really need anything else.

[@vladmartian]

@@Codewrinkles Ok I understand now. Thanks

[@semen083]

What the point is using static constructor for Product class instead of regular? The no way to use validation in former?

[@Codewrinkles]

It is not a static constructor. It is a static method. If you take a careful look you'll notice that the constructor is private. This means that nobody outside the class is able to use the constructor. The only way you can instantiate a new Product is throigh the static method. And there you can fully control state. Also all setters are private. So, by doing ths we fully encapsulate state inside the class itself. That's a common practice in DDD.

[@christianista]

I have an API, the validation message should be returned in several languages. In the Person class there is a property Language, how can I return the error messages in the right language (fr, en, de)?

[@Codewrinkles]

Out of the top of my mind, the best way to achieve this is by using Globalization and Localization: learn.microsoft.com/en-us/aspnet/core/fundamentals/localization?view=aspnetcore-7.0 The core concept is that when you are adding the error messages you don't actually add a string, but add the string through a key to your .resx files.

[@christianista]

@@Codewrinkles I already read this but I may be use it wrong. But