EnvelopeGenerator/EnvelopeGenerator.Application/Common/Behaviors/ValidationBehavior.cs

using DigitalData.Core.Exceptions;
using FluentValidation;
using MediatR;

namespace EnvelopeGenerator.Application.Common.Behaviors;

public class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : notnull
{
    private readonly IEnumerable<IValidator<TRequest>> _validators;

    public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
    {
        _validators = validators;
    }

    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken)
    {
        if (_validators.Any())
        {
            var context = new ValidationContext<TRequest>(request);
            var failures = _validators
                .Select(v => v.Validate(context))
                .SelectMany(r => r.Errors)
                .Where(f => f is not null)
                .ToList();

            if (failures.Count > 0)
            {
                var message = string.Join("; ", failures.Select(f => f.ErrorMessage).Distinct());
                throw new BadRequestException(message);
            }
        }

        return await next();
    }
}