Autofac and Pipeline Behavior

I am trying to get new pipeline working with autofac but it is running the same pipeline many times with different types.

Adding the following class the MediatR example

public class BehaviorOne<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse> 
    {

        private readonly TextWriter _writer;

        public BehaviorOne(TextWriter writer)
        {
            _writer = writer;
        }

        public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next)
        {
            //System.Diagnostics.Trace.WriteLine($"BehaviorOne - Handle {typeof(TRequest).Name}");

            _writer.WriteLine("BehaviorOne - Handle " + typeof(TRequest).Name);

            var response = await next();

            return response;
        }
    }

Added this to the BuildMediator for the autofac example program.cs

builder.RegisterGeneric(typeof(BehaviorOne<,>)).As(typeof(IPipelineBehavior<,>));

Produced the following output:

Sample mediator implementation using send, publish and post-request handlers in sync and async version.
---------------
Sending Ping...
BehaviorOne - Handle Ping
BehaviorOne - Handle Object
BehaviorOne - Handle IRequest`1
Received: Ping Pong
Sending Ping async...
BehaviorOne - Handle PingAsync
BehaviorOne - Handle Object
BehaviorOne - Handle IRequest`1
Received async: Ping Pong
Publishing Pinged...
Got pinged also.
Got pinged.
Got notified.
Got notified also async.
Publishing Pinged async...
Got notified.
Got pinged also async.
Got pinged async.
Got notified also async.

For the structure map example you get:

Sample mediator implementation using send, publish and post-request handlers in sync and async version.
---------------
Sending Ping...
BehaviorOne - Handle Ping
Received: Ping Pong
Sending Ping async...
BehaviorOne - Handle PingAsync
Received async: Ping Pong
Publishing Pinged...
Got notified.
Got pinged.
Got pinged also.
Got notified also async.
Publishing Pinged async...
Got notified.
Got notified also async.
Got pinged async.
Got pinged also async.

[seroche]

Hey guys,

I had the same problem this morning and I solved by implementing my own ScopedContravariantRegistrationSource (as advised by jdaigle). It’s a simplistic (probably incomplete) implementation but it works quite well with my easy use case:

Here is the code:

    public class ScopedContravariantRegistrationSource : IRegistrationSource
    {
        private readonly IRegistrationSource _source = new ContravariantRegistrationSource();
        private readonly List<Type> _types = new List<Type>();

        public ScopedContravariantRegistrationSource(params Type[] types)
        {
            if (types == null)
                throw new ArgumentNullException(nameof(types));
            if (!types.All(x => x.IsGenericTypeDefinition))
                throw new ArgumentException("Supplied types should be generic type definitions");
            _types.AddRange(types);
        }

        public IEnumerable<IComponentRegistration> RegistrationsFor(
            Service service,
            Func<Service, IEnumerable<IComponentRegistration>> registrationAccessor)
        {
            var components = _source.RegistrationsFor(service, registrationAccessor);
            foreach (var c in components)
            {
                var defs = c.Target.Services
                    .OfType<TypedService>()
                    .Select(x => x.ServiceType.GetGenericTypeDefinition());

                if (defs.Any(_types.Contains))
                    yield return c;
            }
        }

        public bool IsAdapterForIndividualComponents => _source.IsAdapterForIndividualComponents;
    }

Then simply call in Startup: then builder.RegisterSource(typeof(MyBehavior<,>));

Regards, Seb

[jdaigle]

Part of the problem with the ContravariantRegistrationSource is that it applies to any service resolved. Instead, we want it limited in scope.

For a project I ended up implementing my own ScopedContravariantRegistrationSource (largely copy/pasted the original source but added code to restrict which services it applied to via constructor parameter). Then register this source for only the services you want to have contravariant resolution.

It’s certainly not as flexible as StructureMap’s configuration, but it works decently well.