ICommandHandler/IQueryHandler with async/await

Question

EDITH says (tl;dr)

I went with a variant of the suggested solution; keeping all ICommandHandlers and IQueryHandlers potentially aynchrono

Answer 1

I created a project for just this - I wound up not splitting commands and queries, instead using request/response and pub/sub - https://github.com/jbogard/MediatR

public interface IMediator
{
    TResponse Send(IRequest request);
    Task SendAsync(IAsyncRequest request);
    void Publish(TNotification notification) where TNotification : INotification;
    Task PublishAsync(TNotification notification) where TNotification : IAsyncNotification;
}

For the case where commands don't return results, I used a base class that returned a Void type (Unit for functional folks). This allowed me to have a uniform interface for sending messages that have responses, with a null response being an explicit return value.

As someone exposing a command, you explicitly opt-in to being asynchronous in your definition of the request, rather than forcing everyone to be async.