MyCustomPipelineValidationBehavior.cs

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

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

    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
    {
        var validationFailures = _validators
            .Select(validator => validator.Validate(request))
            .SelectMany(validationResult => validationResult.Errors)
            .Where(validationFailure => validationFailure != null)
            .ToList();

        if (validationFailures.Any())
        {
            var error = string.Join("\r\n", validationFailures);
            throw new MyCustomValidationException(error);
        }

        return next();
    }
}