Commands can validate their input in CQS. What they don't do, in strict CQS, is return values. They can set state which can then be queried after execution whic

Commands can validate their input in CQS. What they don't do, in strict CQS, is return values. They can set state which can then be queried after execution which can let you retrieve an updated result or check to see if an error occurred during execution or whatever.