Add validation to a MediatR behavior pipeline?

The process is exactly the same, you just have to change the interface to use the new IPipelineBehavior<TRequest, TResponse> interface.

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 Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next)
    {
        var context = new ValidationContext(request);
        var failures = _validators
            .Select(v => v.Validate(context))
            .SelectMany(result => result.Errors)
            .Where(f => f != null)
            .ToList();

        if (failures.Count != 0)
        {
            throw new ValidationException(failures);
        }

        return next();
    }
}

For the validators, you should register all the validators as IValidator<TRequest> in the built-in container so they'll be injected in the behavior. If you don't want to register them one by one, I suggest that you have a look at the great Scrutor library that brings assembly scanning capabilities. This way it'll find your validators itself.

Also, with the new system, you don't use the decorator pattern anymore, you just register your generic behavior in the container and MediatR will pick it up automatically. It could look something like:

var services = new ServiceCollection();
services.AddMediatR(typeof(Program));
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));
var provider = services.BuildServiceProvider();

On the new version (MediatR (>= 9.0.0)) you can do something like this:

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 Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
    {
        var context = new ValidationContext<TRequest>(request);
        var failures = _validators
            .Select(v => v.Validate(context))
            .SelectMany(result => result.Errors)
            .Where(f => f != null)
            .ToList();

        if (failures.Count != 0)
        {
            throw new ValidationException(failures);
        }

        return next();
    }
}

Remember to add var context = new ValidationContext<TRequest>(request); in previous version like FluentApi 8.0 or below it used something like this var context = new ValidationContext(request);

for Register in Asp.Net Core in under IServiceCollection wright below code:

services.AddMediatR(Assembly.GetExecutingAssembly());
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));

Hope that's helpful!

I've packed .net core integration into nuget, feel free to use it: https://www.nuget.org/packages/MediatR.Extensions.FluentValidation.AspNetCore

Just insert in configuration section:

services.AddFluentValidation(new[] {typeof(GenerateInvoiceHandler).GetTypeInfo().Assembly});

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