GravatarBen Ramey's Blog
Scripture, programming problems, solutions and stories.

You Can't Subscribe to IEvent with NServiceBus

The reason you might want to subscribe to IEvent is pretty straightforward. I wanted to do it for the simple reason of wanting to be able to choose (based on configuration) whether I wanted to notify anyone of any event that could be published on my bus.

In other words, whenever an event of any type was published, I wanted one particular endpoint to subscribe to that event and then decide whether it should do anything with it.

I thought the code to do that should be pretty simple. Since all published events should implement the IEvent interface and NServiceBus supports polymorphism in its message handling, creating a message handler that implemented IHandleMessages<IEvent> should be all I should have needed!

I was testing this with only partial success and so I thought it was working.

public class EventHandler : IHandleMessages<IEvent>
   public void Handle(IEvent message)

My Partial Success

The reason I had partial success was because I was also explicitly handling other events in the same endpoint on a special saga I had setup. So, when the endpoint started up, it would subscribe to these events. At first, I only wanted to send notifications for these events anyway, so it all seemed to be working. My special IEvent would work because the Saga subscribed this same endpoint to those few events explicitly.

My Full Failure

I realized I had only partially succeeded when I started wanting to send notifications for other events that were not in that Saga. I dug into the issue for a few hours and finally began to wonder if NServiceBus (or maybe the RabbitMQ transport) was explicitly ignoring IEvent when it setup subscriptions.

Sure enough, it does.

To build a list of types to subscribe to, NServiceBus uses the Conventions class in the NServiceBus.Core namespace. One of the checks that code does is to filter the list of potential types with the IsEventType method. This method checks if the Type is in a Particular (NServiceBus) DLL. See the code here: IsEventType

My Solution

The solution is simple enough. Instead of listening to IEvent, listen to a custom interface that you implement on all of your events. This is probably more close to what you are trying to do anyway--listen to all events that your system produces.

So, I simply created a marker interface called ICustomEvent.

public interface ICustomEvent : IEvent { }

Then, on all of my bus event classes, instead of implementing IEvent directly, I implemented ICustomEvent.

My handler then looked like this.

public class EventHandler : IHandleMessages<ICustomEvent>
   public void Handle(ICustomEvent message)

Now, the right subscriptions are all set up and my single handler gets every single event published by my entire bus.