Creating complex validation rules with FluentValidation

In this recipe, we will learn how to create complex validation rules using the FluentValidation library. FluentValidation offers a more fluent and flexible way to define validation logic.

Getting ready

We are going to continue working with the same DTO. However, all the validation rules have been removed – both the simple data annotations and the rules created via IValidateObject. The code for this project resides here: chapter02/FluentValidation.

How to do it…

1. Open the project in Visual Studio Code.
2. Open the terminal and type dotnet run.
3. Open the browser and go to http://localhost:5217/scalar/chapter2.

   You have a basic API with some DataAnnotations and a custom DataAnnotations attribute on the DTO.

4. Install the FluentValidation NuGet package:

   dotnet add package FluentValidation.AspNetCore

5. Note that our DTO currently has no validation rules. Add the following code to the EventRegistrationDTO...