[Question]How to write validator for collection model binding in ASP.NET Core?

System Details

• FluentValidation version: FluentValidation.AspNetCore 8.6.2
• Web Framework version: ASP.NET Core 2.2

Issue Description

Asp.Net Core support collection binding like this

[ApiController]
public class ClassroomController : ControllerBase {
	
	[HttpPost]
	public IActionResult CreateFromStudent([FromBody]IEnumerable<Student> students) {
                  // Code for create Classroom
	}
}

Let say my Student will look like this

public class Student 
{
    public Teacher Teacher{get;set;}
    //Some additional properties
}

Now want to validate that all students in IEnumerable<Student> students has the same teacher. The documentation for Collection validator https://docs.fluentvalidation.net/en/latest/collections.html seems not cover this scenario. How do I write a validator to accomplish this without creating a wrapper complex type with the collection as a property?

[JeremySkinner]

Hi, this isn’t something that should be done in the ServiceProviderValidatorFactory, but rather is something you should set up explicitly with your container registrations.

You’re correct that if your action parameter is an IEnumerable<T>, then MVC will instantiate a List<T> - this is because MVC’s model binding process doesn’t actually know your intent when you specify an interface as an action method parameter, so it guesses and creates a List, which is the type that’s then passed into the validation infrastructure.

Personally I’d always recommend using explicit concrete types as action method parameters because of this (ie explicitly specify either List<T> or T[]). If your validator type directly matches the type of the action parameter (ie both are using List), then this will all just work without any issues.

However if you want to continue to use IEnumerable in your validator then you’ll need to be explicit about this in your container registration. The T in AbstractValidator<T> is contravariant, so you should be able to register a mapping with the container for IValidator<List<Student>> to StudentValidator (where StudentValidator inherits AbstractValidator<IEnumerable<T>>). If you set this up with an explicit call to services.AddTransient in your application startup routine then I believe this should work fine.

[lamLeX]

Hi @JeremySkinner , Although the code is compiled for the Validator and registered in the DI, the validation is not triggered upon model binding validation stage. I observed this behavior by setting the breakpoint in the delegate passed to Must()

The code passed the validation and reached the code thats run after validation middleware although the input is invalid

After some debugging and troubleshooting, I found out that my model is actually type of List<T> instead of IEnumerable<T>. Due to the nature of Microsoft DI, it only matches the exact type. The FluentValidationClientModelValidatorProvider only have IValidator<IEnumerable<T>> instead of IValidator<List<T>>; hence, it couldn’t find my validator for List<T>

I wonder if we could do anything in ServiceProviderValidatorFactory.CreateInstance() to accommodate this scenario?