Int Callbacks: Could not find metadata for 'System.Int32'

I know callbacks or RPC calls in general are a big no-no. But let’s say I want to use one anyway as explained here, sending a command and waiting for the reply in a thread that is NOT an EventHandler (I’m aware of deadlocks when doing it in an EventHandler).

I added NServiceBus.Callbacks 3.0.0 nuget, along with calling EnableCallbacks(); in the configuration.

I launch the command with:

var result = await _bus.Request<int>(message).ConfigureAwait(false);
Where _bus is of type IMessageSession

And the EventHandler of the message ends with:
return context.Reply(-1);

When calling the Reply() method, it seems to have difficulty serializing the Int32 reply because it’s not implementing NServiceBus message/event/command types and get this exception:

System.Exception: Could not find metadata for 'System.Int32'.
Ensure the following:
1. 'System.Int32' is included in initial scanning. 
2. 'System.Int32' implements either 'IMessage', 'IEvent' or 'ICommand' or alternatively, if you don't want to implement an interface, you can use 'Unobtrusive Mode'.
   at NServiceBus.Unicast.Messages.MessageMetadataRegistry.GetMessageMetadata(Type messageType)
   at NServiceBus.SerializeMessageConnector.SerializeEnclosedMessageTypes(Type messageType)
   at NServiceBus.SerializeMessageConnector.<Invoke>d__1.MoveNext()

Strangely enough, this code WAS working in the past. I’m not sure which change we did that began to cause this error.

Any way for Int32 to be accepted as a valid message type for using the callback sample code?

This is definitely supported, I ran the our sample that demonstrates this on my end but couldn’t reproduce the issue. Can you try https://docs.particular.net/samples/callbacks/ on your end and see if it work? (If yes perhaps compare the code to what you have to see if there are some differences)

I’m sorry, I missed calling EnableCallbacks on the replying endpoint. Should have double checked my config.