FluentValidation version

11.8.1

ASP.NET version

(ASP).NET Core 8

Summary

I honestly don't know if it is an issue, but it manifested as one to me when it occured.

2 (but actually any amount of) calls to AddValidatorsFromAssemblyContaining(<assembly>) would register every validator in the matching assembly 2 times. Clients, that retrieve something like IEnumerable<IValidator<TTarget>> via DI would get 2 instances of the same validator, potentially leading to executing it twice. Or thrice, or ... you get the point.

What sounds as an exotic use case in the first place, isn't too exotic for applications that make use of Vertical Slice Architecture. There the following structures are quite common:

• Feature A
  -- Validators of Feature A (registered via FeatureA.ServiceCollectionExtensions.AddFeatureAServices)
• Feature B
  -- Validators of Feature B (registered via FeatureA.ServiceCollectionExtensions.AddFeatureBServices)
• Feature C
  -- ...

It basically boils down to the registrations in

But as said, I can't tell if this is an inteded behavior for a use case that I currently cannot see. In that case a parameter for example could provice both options:

AddValidatorsFromAssemblyContaining(..., allowDuplicates: false) {
    if (allowDuplicates)
        services.Add(...)
    else
        services.TryAdd(...)
}

Steps to Reproduce

public class ArbitraryTarget{}

public class ArbitraryValidator : AbstractValidator<ArbitraryTarget> { ... }

//Program.cs
services.AddValidatorsFromAssemblyContaining<Program>();
services.AddValidatorsFromAssemblyContaining<Program>();

//Somewhere.cs
var provider = services.BuildServiceProvider();
var validators = provider.GetServices<IValidator<ArbitraryTarget>>();
//Inspect validators.Count();