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    <p>The pros; composability:</p> <ul> <li>callers can add filters</li> <li>callers can add paging</li> <li>callers can add sorting</li> <li>etc</li> </ul> <p>The cons; non-testability:</p> <ul> <li>Your repository is no longer properly unit testable; you can't rely on a: it working, b: <strong>what</strong> it does; <ul> <li>the caller could add a non-translatable function (i.e. no TSQL mapping; breaks at runtime)</li> <li>the caller could add a filter/sort that makes it perform like a dog</li> </ul></li> <li>Since callers expect <code>IQueryable&lt;T&gt;</code> to be composable, it rules out non-composable implementations - or it forces you to write your own query provider for them</li> <li>it means you can't optimize / profile the DAL</li> </ul> <p>For stability, I've taken to <strong>not</strong> exposing <code>IQueryable&lt;T&gt;</code> or <code>Expression&lt;...&gt;</code> on my repositories. This means I know how the repository behaves, and my upper layers can use mocks without worrying "does the actual repository support this?" (forcing integration tests).</p> <p>I still use <code>IQueryable&lt;T&gt;</code> etc <em>inside</em> the repository - but not over the boundary. I posted some <a href="http://marcgravell.blogspot.com/2009/02/pragmatic-linq.html" rel="noreferrer">more thoughts on this theme here</a>. It is just as easy to put paging parameters on the repository interface. You can even use extension methods (on the interface) to add <em>optional</em> paging parameters, so that the concrete classes only have 1 method to implement, but there may be 2 or 3 overloads available to the caller.</p>
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    1. POTo return IQueryable<T> or not return IQueryable<T>
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    1. USMarc Gravell
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    1. CO@Marc, what would you tell about IEnumerable<T> for repository methods?
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      1. USKonstantin Tarkus
    2. COIf used appropriately, `IEnumerable<T>` would be OK - but mainly for large data. For regular queries, I'd prefer a closed set (array/list/etc). In particular, I'm using MVC at the moment, and I want the **controller** to fetch all data - not defer it until the view - else you can't fully test...
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      1. USMarc Gravell
    3. CO...the controller, as you haven't actually proven it fetching data (since `IEnumerable<T>` is usually used with deferred execution). If the repo returns IList<T> (or similar), then you **know** you have got the data.
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