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    <blockquote> <p>I've read the "blue book" of DDD (Evans) as well as Nilsson and am not clear what approach to validation I should take.</p> </blockquote> <p>Blue book approaches the problem from a different angle. I think that the term 'Validation' is not used because it is a dangerous <a href="http://lostechies.com/jimmybogard/2009/02/15/validation-in-a-ddd-world/" rel="nofollow noreferrer">overgeneralization</a>. A better approach is to think about <strong>object invariants</strong>, not validation. Objects (not only in DDD) should enforce their internal invariants themselves. It is not UI or services or contracts or mappers or 'validation frameworks' or anything else that is external to the objects. Invariants are enforced internally. You may find these answers helpfull: <a href="https://stackoverflow.com/questions/7093819/should-domain-objects-be-allowed-to-be-temporarily-invalid-and-how-does-that-dec/7096430#7096430">1</a>, <a href="https://stackoverflow.com/questions/6966288/ddd-client-side-validation/7019985#7019985">2</a>, <a href="https://stackoverflow.com/questions/7132468/how-to-enforce-constraint-such-as-any-field-or-specific-field-must-not-change-i/7133121#7133121">3</a>, <a href="https://stackoverflow.com/questions/7139831/bl-services-exception-or-method-result/7141395#7141395">4</a>.</p> <blockquote> <p>I could also put guards in the property setters that prevent unacceptable values from ever putting the entity into an invalid state. This would mean that AutoMapper would fail when trying to map an invalid value.</p> </blockquote> <p>You probably should not care about AutoMapper failing or using AutoMapper at all. Domain objects should encapsulate and enforce their internal invariants and throw exception if the attempt to break it is made. It is very simple and you should not compromise the simplicity and expressiveness of your domain objects because of some infrastructural issues. The goal of DDD is not to satisfy AutoMapper's or any other framework's requirements. If the framework does not work with your domain objects don't use it.</p>
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    1. POValidation with DDD in SOA application using IoC
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    1. COThe links are great. It's clear to me now that the interactive-type of validation that allows the object to go into an invalid state and provides a list of errors (via IDataErrorInfo, for instance) belongs in the UI/Presentation layer (ViewModel). And I'm good with putting guards in my domain objects to prevent them from going into an invalid state. But are you advocating against also putting checks in the service and/or facade layer unless they span multiple entities/aggregates?
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    2. COYou can put additional checking into services and UI as needed. My point is that domain objects should not rely on these checks.
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    3. CO@SonOfPirate - Just as a matter of interest, I use CodeContracts to enforce invariants in my domain objects.
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