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    <p>I was looking for an answer to a similar question and agree with the two highest-ranked answers. Trying to clarify this for myself, I found that <strong>if</strong> Specifications, which go hand-in-hand with the Repository pattern, are implemented as first-class members of the domain model, then I can</p> <ul> <li><strong>reuse</strong> Specification definitions with different parameters,</li> <li><strong>manipulate</strong> existing Specification instances' parameters (e.g. to specialize),</li> <li><strong>combine</strong> them,</li> <li><strong>perform business logic</strong> on them without ever having to do any database access,</li> <li>and, of course, <strong>unit-test</strong> them independent of actual Repository implementations.</li> </ul> <p>I may even go so far and state that <em>unless</em> the Repository pattern is used together with the Specification pattern, it's not really "Repository," but a DAL. A contrived example in pseudo-code:</p> <pre><code>specification100 = new AccountHasMoreOrdersThan(100) specification200 = new AccountHasMoreOrdersThan(200) assert that specification200.isSpecialCaseOf(specification100) specificationAge = new AccountIsOlderThan('2000-01-01') combinedSpec = new CompositeSpecification( SpecificationOperator.And, specification200, specificationAge) for each account in Repository&lt;Account&gt;.GetAllSatisfying(combinedSpec) assert that account.Created &lt; '2000-01-01' assert that account.Orders.Count &gt; 200 </code></pre> <p>See <a href="http://martinfowler.com/apsupp/spec.pdf" rel="nofollow noreferrer">Fowler's Specification Essay</a> for details (that's what I based the above on).</p> <p>A DAL would have specialized methods like</p> <pre><code>IoCManager.InstanceFor&lt;IAccountDAO&gt;() .GetAccountsWithAtLeastOrdersAndCreatedBefore(200, '2000-01-01') </code></pre> <p>You can see how this can quickly become cumbersome, especially since you have to define each of the DAL/DAO interfaces with this approach <em>and</em> implement the DAL query method.</p> <p>In .NET, LINQ queries <em>can</em> be one way to implement specifications, but combining Specification (expressions) may not be as smooth as with a home-grown solution. Some ideas for that are described in <a href="https://stackoverflow.com/questions/457316">this SO Question</a>.</p>
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