CustomAsync method does not change ModelState

I am using Asp.Net Core V3. My code looks like below:

RuleFor(x => x.ApplicationName)
    .NotEmpty().WithMessage("Application name is required to continue")
    .CustomAsync(async (appName, context, cancellationToken) =>
    {
        var application = await applicationProvider.GetLatestApplicationWithContextValuesAsync(appName, false);
        if(application != null)
            context.AddFailure($"Application with name: {appName} already exists.");
    });

I am able to debug the app and see that context.AddFailure is executed but the ModelState does not reflect the validation error.

Is this intentional or a bug ?

When some validation fail while using RuleFor the ModelState change is reflected.

[JeremySkinner]

Alternatively, you can leave automatic validation enabled (if you want it in some other places) and instead use [CustomizeValidator(Skip=true)] on the parameter declaration (see https://docs.fluentvalidation.net/en/latest/aspnet.html#validator-customization) to skip auto validation for specific instances.

[JeremySkinner]

Did not check in details how it works but my feeling is that if you call AddFluentValidation() method there is no way how to stop automatic validation from happening

Calling AddFluentValidation is what enables automatic validation. If you don’t want automatic validation, then don’t call AddFluentValidation in your startup. Instead just register the services you need directly with the service provider:

// Don't call AddFluentValidation, as this enables automatic validation.
services.AddControllers().ConfigureApiBehaviorOptions(...);

// Register the services you need.
services.AddTransient<IValidatorFactory, ServiceProviderValidatorFactory>();
services.AddControllresFromAssemblyContaining<MyValidator>();

Note that by switching to manual validation like this you also lose clientside validation integration (but this is no big loss as it’s very limited anyway). Also be aware that ASP.NET’s model binding process will also still generate some error messages prior to FluentValidation being invoked (eg if you omit a value for a non-nullable value type).