Single Message Handler Factory using Autofac

Question

So I have a marker interface called IMessage.Then I have classes like:

public class MessageA: IMessage 
{
}

Then I have message handlers defined as:

internal interface IMessageHandler<in T> where T: IMessage
{
    void Handle(T message);
}

public class MessageAHandler : IMessageHandler<MessageA> 
{
    public void Handle(T message)
    { 
        //Some logic here
    }
}

I want to re-route these messages that I get to the corresponding message handlers when I get a new message. Example:

public class MessageReceiver
{
    public void ReceiveMessage(IMessage message)
    {
        //somehow resolve the appropiate message handler here

        messageHandler.Handle(message);
    }
}

I can accomplish this right now by using factories like below but I need to have a dependency on each a new factory per different type of message. So I'm wondering if there is a way to create a single factory that will be smart enough to resolve the appropiate message handler?

public class MessageReceiver
{
   private readonly Func<IMessageHandler<MessageA>> _messageAFactory;

   public MessageReceiver(Func<IMessageHandler<MessageA>> messageAFactory)
   {
       _messageAFactory= messageAFactory;
   }

   public void ReceiveMessage(IMessage message)
   {
       if (message is MessageA)
       {
           var messageHandler = _messageAFactory();
           messageHandler.Handle(message as MessageA);
       }

       // Add more if-statements here for more messages
   }
}

---

Autofac Registration

public class InfrastructureModule : Module
{
    protected override void Load(ContainerBuilder builder)
    {
        //Register the types in the infrastructure assembly
        builder.RegisterAssemblyTypes(ThisAssembly).AsImplementedInterfaces()
            .InstancePerLifetimeScope();

        //Register the message handlers
        builder.RegisterAssemblyTypes(ThisAssembly)
        .Where(x => x.IsAssignableFrom(typeof(IMessageHandler<IMessage>)))
        .InstancePerDependency().AsImplementedInterfaces();
    }
}

Answer 1

First I'll make a small implementation for your messages, just a basic handled flag, in order to test

public class MessageA : IMessage
{
    public bool Handled
    {
        get;
        private set;
    }

    public void MarkAsHandled()
    {
        this.Handled = true;
    }
}

public class MessageB : IMessage
{
    public bool Handled
    {
        get;
        private set;
    }

    public void MarkAsHandled()
    {
        this.Handled = true;
    }
}

Now let's implement both handlers as:

public class MessageAHandler : IMessageHandler<MessageA>
{
    public void Handle(MessageA message)
    {
        message.MarkAsHandled();
    }
}

public class MessageBHandler : IMessageHandler<MessageB>
{
    public void Handle(MessageB message)
    {
        message.MarkAsHandled();
    }
}

As a side note, you might want to mark your IMessageHandler interface as public (I get compiler error if visibility is set as internal).

Now let's add a small handler:

public interface IMessageHandler
{
    Type MessageType { get; }
    void Handle(IMessage message);
}

public class MessageHandlerAdapter<T> : IMessageHandler where T : IMessage
{
    private readonly Func<IMessageHandler<T>> handlerFactory;

    public MessageHandlerAdapter(Func<IMessageHandler<T>> handlerFactory)
    {
        this.handlerFactory = handlerFactory;
    }

    public void Handle(IMessage message)
    {
        var handler = handlerFactory();
        handler.Handle((T)message);
    }

    public Type MessageType
    {
        get { return typeof(T); }
    }
}

We can now implement MessageReceiver this way:

public class MessageReceiver
{
    private readonly IEnumerable<IMessageHandler> handlers;

    public MessageReceiver(IEnumerable<IMessageHandler> handlers)
    {
        this.handlers = handlers;
    }

    public void ReceiveMessage(IMessage message)
    {
        var handler = this.handlers.Where(h => h.MessageType == message.GetType()).FirstOrDefault();

        if (handler != null)
        {
            handler.Handle(message);
        }
        else
        {
            //Do something here, no handler found for message type
        }
    }
}

Now to test that our messages are processed properly, here is a small test:

[TestClass]
public class TestSelector
{
    private IContainer container;

    [TestMethod]
    public void TestMethod()
    {
        var processor = container.Resolve<MessageReceiver>();

        MessageA ma = new MessageA();
        MessageB mb = new MessageB();

        processor.ReceiveMessage(ma);
        processor.ReceiveMessage(mb);

        Assert.AreEqual(ma.Handled, true);
        Assert.AreEqual(mb.Handled, true);

    }
}

And we need to modify registration a bit, if opting for manual registration, we do as follow:

    public TestSelector()
    {
        var containerBuilder = new ContainerBuilder();

        containerBuilder.RegisterType<MessageAHandler>().As<IMessageHandler<MessageA>>();
        containerBuilder.RegisterType<MessageBHandler>().As<IMessageHandler<MessageB>>();

        containerBuilder.RegisterType<MessageHandlerAdapter<MessageA>>().As<IMessageHandler>();
        containerBuilder.RegisterType<MessageHandlerAdapter<MessageB>>().As<IMessageHandler>();

        containerBuilder.RegisterType<MessageReceiver>();

        this.container = containerBuilder.Build();
    }

In here, we now need to register one handler and the relevant adapter.

It is also of course possible to perform assembly scan, but this requires a little bit more plumbing, since using:

builder.RegisterAssemblyTypes(ThisAssembly)
    .Where(x => x.IsAssignableFrom(typeof(IMessageHandler<IMessage>)))
    .InstancePerDependency().AsImplementedInterfaces();

will not work

typeof(MessageAHandler).IsAssignableFrom(typeof(IMessageHandler<IMessage>))

will return false, since MessageAHandler implements IMessageHandler, not IMessageHandler

To do automatic discovery and registration, here is a snippet:

    public TestSelector()
    {
        var containerBuilder = new ContainerBuilder();

        Func<Type, Type> GetHandlerInterface = (t) => t.GetInterfaces()
            .Where(iface => iface.IsGenericType && iface.GetGenericTypeDefinition() == typeof(IMessageHandler<>)).FirstOrDefault();

        var handlerTypes = typeof(IMessage).Assembly.GetTypes()
            .Where(type => type.IsClass
                && !type.IsAbstract
                && GetHandlerInterface(type) != null);

        foreach (Type handlerType in handlerTypes)
        {
            Type messageType = GetHandlerInterface(handlerType).GetGenericArguments()[0];
            var genericHandler = typeof(MessageHandlerAdapter<>).MakeGenericType(messageType);

            containerBuilder.RegisterType(handlerType).AsImplementedInterfaces();
            containerBuilder.RegisterType(genericHandler).As<IMessageHandler>();
        }

        containerBuilder.RegisterType<MessageReceiver>();

        this.container = containerBuilder.Build();
    }

Answer 2

For anyone who is still looking for better solution for auto dispatching to appropriate message handlers registered, there is a nice implementation via MediatR.This is awesome library which can dispatch messages to appropriate registered handlers, and has capability to post messages to multiples handlers. It is best suited for CQRS scenarios and also for Async Web API, refer CQRS using MediatR . There is a nice support when using DI container like Autofac and StructuredMap, Refer to wiki page of MediatR wiki for full details on DI support.