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    Please see this webpage: enterprisecraftsmanship.com/2016/09/13/validation-and-ddd. Would you suggest something like: Execute / CanExecute pattern for the validation? Commented Jun 27, 2017 at 13:18
  • I have also injected RulesOfferCalculator into this class. What do you think of that? I believe it follows the open close principle. Commented Jun 27, 2017 at 13:19
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    "Would you suggest something like: Execute / CanExecute pattern for the validation?". No! Non-atomic "test then set" operations like this are an anti-pattern as, unless you have exactly one copy of the application, operating on a single thread, then the state can change between the "test" and the "set". So use either an "execute and throw" pattern or a "try execute" pattern that returns success or the error result. Commented Jun 27, 2017 at 15:19
  • @David Arno, thanks. The author of the article states that: TryExecute is problematic because it breaks the CQS principle. Do you not agree? Also do you mean return the word: "success" as a string? Commented Jun 29, 2017 at 12:52
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    @w0051977, it probably won't surprise you to discover that I have as little time for CQS as for RDM. CQS creates unnecessary complexity and is generally a nightmare in multi-threaded situations. I prefer my commands to return a success or an error result. Commented Jun 29, 2017 at 16:10