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
IHandleMessages<IEvent> should be all I should have needed!
I was testing this with only partial success and so I thought it was working.
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
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
The solution is simple enough. Instead of listening to
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
So, I simply created a marker interface called
Then, on all of my bus event classes, instead of implementing
My handler then looked like this.
Now, the right subscriptions are all set up and my single handler gets every single event published by my entire bus.