ReC/src/ReC.Application/Common/Behaviors/ValidationBehavior.cs

using FluentValidation;
using MediatR;

namespace ReC.Application.Common.Behaviors;

public class ValidationBehavior<TRequest, TResponse>(IEnumerable<IValidator<TRequest>> validators) : IPipelineBehavior<TRequest, TResponse>
    where TRequest : notnull
{
    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancel)
    {
        if (validators.Any())
        {
            var context = new ValidationContext<TRequest>(request);

            var validationResults = await Task.WhenAll(
                validators.Select(v =>
                    v.ValidateAsync(context, cancel)));

            var failures = validationResults
                .SelectMany(r => r.Errors)
                .Where(f => f != null)
                .ToList();

            if (failures.Count != 0)
                throw new ValidationException(failures);
        }
        return await next(cancel);
    }
}