Question

Problem

I have a protobuf message definition with a MessageType field, which is an enum. Given an incoming protobuf message, I would like to resolve some IMessageHandlers from an IoC container based on the MessageType. The problem is twofold: How do I express the MessageType constraint when writing an IMessageHandler, and how do I resolve only the desired handlers from the IoC container?

I’m using Autofac but am interested in hearing solutions for any container.

My thoughts:

For expressing the constraint, I see two options: a property or an attribute (can’t use generics because it is an enum value). I like the property because it makes it impossible to write an IMessageHandler without specifying the constraint, but the downside is that it has to be instantiated before you can see the property value.

In my current code, I’m using the property approach. I resolve all IMessageHandlers and do the filtering manually, but it seems like there should be a better way. Not that I’m too worried about performance, it just kind of smells that I’m resolving instances that don’t get used.

Update

To make it a little more clear, here’s what I’m doing now

public interface IMessageHandler
{
    MessageType TargetType { get; }

    void Handle(IMessage message);
}

public class SomeHandler : IMessageHandler
{
    public MessageType TargetType
    {
        get { return MessageType.Type1;  }
    }

    public void Handle(IMessage message)
    {
        // implementation
    }
}

So when I receive a protobuf message, I resolve all IMessageHandlers and invoke the ones whose TargetType matches the MessageType of the incoming message.

Answer 1

I realized that Autofac’s Keyed Services can help me out. I think I’m going to go with this approach.

1. Use an attribute to declare what MessageType a given IMessageHandler is interested in.
2. Register every IMessageHandler keyed to the MessageType
3. Use ResolveKeyed to get only the IMessageHandlers I’m interested int.

The nice thing is if someone forgets to use the attribute, we can catch it while building the container. Here’s a full example. Any suggestions are most welcome!

class Program
{
    static IContainer container;

    static void Main(string[] args)
    {
        var builder = new ContainerBuilder();
        builder.RegisterAssemblyTypes(typeof(Program).Assembly)
            .AssignableTo<IMessageHandler>()
            .Keyed<IMessageHandler>(t => GetMessageType(t));

        container = builder.Build();

        InvokeHandlers(MessageType.Type1);
        InvokeHandlers(MessageType.Type2);

        Console.ReadKey();
    }

    static MessageType GetMessageType(Type type)
    {
        var att = type.GetCustomAttributes(true).OfType<MessageHandlerAttribute>().FirstOrDefault();
        if (att == null)
        {
            throw new Exception("Somone forgot to put the MessageHandlerAttribute on an IMessageHandler!");
        }

        return att.MessageType;
    }

    static void InvokeHandlers(MessageType type)
    {
        using (var lifetime = container.BeginLifetimeScope())
        {
            // I'm impressed that Autofac knows what I mean here!
            var handlers = lifetime.ResolveKeyed<IEnumerable<IMessageHandler>>(type);
            foreach (var handler in handlers)
            {
                handler.Handle();
            }
        }
    }
}

public enum MessageType
{
    Type1,
    Type2,
}

public interface IMessageHandler
{
    void Handle();
}

public class MessageHandlerAttribute : Attribute
{
    public MessageHandlerAttribute(MessageType messageType)
    {
        MessageType = messageType;
    }

    public MessageType MessageType { get; private set; }
}

[MessageHandler(MessageType.Type1)]
public class Handler1 : IMessageHandler
{
    public void Handle()
    {
        Console.WriteLine("A handler for Type1");
    }
}

[MessageHandler(MessageType.Type1)]
public class Handler2 : IMessageHandler
{
    public void Handle()
    {
        Console.WriteLine("Another handler for Type1");
    }
}

[MessageHandler(MessageType.Type2)]
public class Handler3 : IMessageHandler
{
    public void Handle()
    {
        Console.WriteLine("A handler for Type2");
    }
}