DocumentService/DocumentService.Application/Common/Behaviors/ValidationBehavior.cs

using FluentValidation;
using MediatR;

namespace DocumentService.Application.Common.Behaviors;

/// <summary>
/// MediatR Pipeline Behavior that validates requests using FluentValidation
/// Executes BEFORE the Handler
/// </summary>
public class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    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)
    {
        // Wenn keine Validators registriert sind, direkt weiter zum Handler
        if (!_validators.Any())
        {
            return await next();
        }

        // Validation Context erstellen
        var context = new ValidationContext<TRequest>(request);

        // Alle Validators parallel ausführen
        var validationResults = await Task.WhenAll(
            _validators.Select(v => v.ValidateAsync(context, cancellationToken))
        );

        // Fehler sammeln
        var failures = validationResults
            .Where(r => !r.IsValid)
            .SelectMany(r => r.Errors)
            .ToList();

        // Bei Fehlern: ValidationException werfen (wird von Exception Middleware gefangen)
        if (failures.Any())
        {
            throw new ValidationException(failures);
        }

        // Validation erfolgreich ? weiter zum Handler
        return await next();
    }
}