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    <p>The first question is the most difficult to answer. Is it bad practice to have Entities depend on outside classes? It's certainly not the most common thing to do.</p> <p>If, for example, you inject a Repository into your Entities you effectively have an implementation of the <a href="https://en.wikipedia.org/wiki/Active_record_pattern" rel="nofollow noreferrer">Active Record pattern</a>. Some people like this pattern for the convenience it provides, while others (like me) consider it a code smell or anti-pattern because it violates the <a href="https://en.wikipedia.org/wiki/Single_responsibility_principle" rel="nofollow noreferrer">Single Responsibility Principle</a> (SRP).</p> <p>You could argue that injecting other dependencies into Entities would pull you in the same direction (away from SRP). On the other hand you are certainly correct that if you don't do this, the pull is towards an <a href="https://martinfowler.com/bliki/AnemicDomainModel.html" rel="nofollow noreferrer">Anemic Domain Model</a>.</p> <p>I struggled with all of this for a long time until I came across Greg Young's (abandonded) <a href="https://cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf" rel="nofollow noreferrer">paper on DDDD</a> where he explains why <strong>the stereotypical n-tier/n-layer architecture will always be CRUDy</strong> (and thus rather anemic).</p> <p>Moving our focus to modeling Domain objects as <strong>Commands and Events</strong> instead of Nouns seems to enable us to build a proper object-oriented domain model.</p> <p>The second question is easier to answer. You can always use an <a href="https://stackoverflow.com/questions/1926826/cant-combine-factory-di/1927167#1927167">Abstract Factory to create instances at run-time</a>. With Castle Windsor you can even use the Typed Factory Facility, relieving you of the burden of implementing the factories manually.</p>
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    1. POWhy not use an IoC container to resolve dependencies for entities/business objects?
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    1. USSteven
    1. USMark Seemann
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    1. COThanks Mark. I've seen the Typed Factory and read your other posts on the Abstract Factory method, but I have never seen any examples of them being used to resolve entities. Is this because most people design their entities without any dependencies except a repository? Am I getting into trouble down the road if I make rigorous use of something like Typed Factory to resolve my entities that have outside dependencies?
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      1. USCasey Wilkins
    2. COThe thing I was trying to say is that if your Entities contains other collaborators that may access other Entities, etc. you may run into all sorts of maintenance issues - not to mention the SRP violations and the N+1 issues. That's why Evans recommends treating each Entity as an Aggregate Root.
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    3. COIn my example ShipQuoter pulls shipping rates for an order from a webservice (e.g. UPS). Would you make this a Service that accepts IOrder, or would you make it part of the domain object like Order.GetRates?
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