How would one apply command query separation (CQS), when result data is needed from a command?

Question

In wikipedia\'s definition of command query separation, it is stated that

More formally, methods should return a value only if they are referentially t

Answer 1

I'm really late to this, but there are a few more options that haven't been mentioned (though, not sure if they are really that great):

One option I haven't seen before is creating another interface for the command handler to implement. Maybe ICommandResult that the command handler implements. Then when the normal command runs, it sets the result on the command result and the caller then pulls out the result via the ICommandResult interface. With IoC, you can make it so it returns the same instance as the Command Handler so you can pull the result back out. Though, this might break SRP.

Another option is to have some sort of shared Store that lets you map results of commands in a way that a Query could then retrieve. For example, say your command had a bunch of information and then had an OperationId Guid or something like that. When the command finishes and gets the result, it pushes the answer either to the database with that OperationId Guid as the key or some sort of shared/static dictionary in another class. When the caller gets back control, it calls a Query to pull back based the result based on the given Guid.

The easiest answer is to just push the result on the Command itself, but that might be confusing to some people. The other option I see mentioned is events, which you can technically do, but if you are in a web environment, that makes it much more difficult to handle.

Edit

After working with this more, I ended up creating a "CommandQuery". It is a hybrid between command and query, obviously. :) If there are cases where you need this functionality, then you can use it. However, there needs to be really good reason to do so. It will NOT be repeatable and it cannot be cached, so there are differences compared to the other two.