JSM.FluentValidation.AspNet.AsyncFilter

FluentValidation documentation states:

You should not use asynchronous rules when using automatic validation with ASP.NET as ASP.NET’s validation pipeline is not asynchronous. If you use asynchronous rules with ASP.NET’s automatic validation, they will always be run synchronously.

This library provides an ASP.NET filter that performs async Validation using FluentValidation.

Archived

This library was created to work around a limitation in FluentValidation's ASP.NET Core integration: the FluentValidation.AspNetCore auto-validation pipeline called Validate() synchronously, which meant async rules such as MustAsync, CustomAsync, and WhenAsync could not execute properly — they would either throw AsyncValidatorInvokedSynchronouslyException (v11+) or silently run synchronously (v10 and earlier), risking deadlocks. This library solved that by providing an IAsyncActionFilter that explicitly called ValidateAsync() on resolved validators before the controller action executed.

Since then, the FluentValidation.AspNetCore package has been deprecated by its maintainer and is no longer updated. FluentValidation itself continues to fully support async validation through its IValidator<T>.ValidateAsync() method — the recommended approach is now to either call ValidateAsync manually in your controllers/services or use an actively maintained auto-validation library such as SharpGrip.FluentValidation.AutoValidation, which is recommended by the official FluentValidation documentation.

This repository is archived because its sole dependency (FluentValidation.AspNetCore) is discontinued, it targets only .NET 6 (which reached end-of-life in November 2024), and the problem it solved is now fully addressed by the alternatives above.

Get Started

To use the filter, first install the NuGet package:

dotnet add package JSM.FluentValidation.AspNet.AsyncFilter

Then, it's necessary to register the filter on the ConfigureServices portion of the Startup.cs.

services
    .AddControllers
    .AddModelValidationAsyncActionFilter();

The next step is to register the validators of the API:

services
    .AddScoped<IValidator<MyClass>, MyClassValidator>>();

Or use automatic registration.

Customization

If also possible to apply the filter only to controllers that contains the ApiControllerAttribute.

    ...
    .AddModelValidationAsyncActionFilter(options =>
    {
        options.OnlyApiController = true;
    });

This way, other controllers requests without the attribute will be ignored.

Contributing

See CONTRIBUTING.md.