Architecture

[mshamrai]

I think that in the domain should be either WebApi, or just some listener (since the request is only one).
The App layer must only depend on the domain interfaces.
There is my suggestion:

Therefore, the user must only subscribe to the event (using the RegisterUserHandler method).

[mshamrai]

Architecture v2.0

The yellow box is representing interface which user must implement and put it into RegisterUserHandler. At this point, Registrator adds a Handle method as an event listener.

[DedSec256]

A good concept. But it seems to me that the dispatch of the response is IAliceHandler's area of ​​responsibility. We can change it signature to Task<AliceResponseShortModel> Handle (AliceArgs)