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    <p>I have used a blend of #2 and #3, but I prefer a strict generic repository if possible (stricter than even suggested in the link for #3). #1 is no good because it plays poorly with unit testing. </p> <p>If you have a smaller domain or need to constrict which entities that your domain allows to be queried, I suppose #2- or #3 that defines entity specific repository interfaces that themselves implement a generic repository- makes sense. However, I find it to be exhausting and unnecessary to write an interface and a concrete implementation for every entity I want to query. What good is <code>public interface IFooRepository : IRepository&lt;Foo&gt;</code> (again, unless I need to constrain developers to a set of allowed aggregate roots)?</p> <p>I just define my generic repository interface, with <code>Add</code>, <code>Remove</code>, <code>Get</code>, <code>GetDeferred</code>, <code>Count</code>, and <code>Find</code> methods (Find returns an <code>IQueryable</code> interface allowing LINQ), create a concrete generic implementation, and call it a day. I rely heavily on <code>Find</code> and thus LINQ. If I need to use a specific query more than once, I use extension methods and write the query using LINQ.</p> <p>This covers 95% of my persistence needs. If I need to perform some sort of persistence action that can't be done generically, I use a home-grown <code>ICommand</code> API. For example, say I'm working with NHibernate and I need to perform a complex query as part of my domain, or perhaps I need to do a bulk command. The API looks roughly like this:</p> <pre><code>// marker interface, mainly used as a generic constraint public interface ICommand { } // commands that return no result, or a non-query public interface ICommandNoResult : ICommand { void Execute(); } // commands that return a result, either a scalar value or record set public interface ICommandWithResult&lt;TResult&gt; : ICommand { TResult Execute(); } // a query command that executes a record set and returns the resulting entities as an enumeration. public interface IQuery&lt;TEntity&gt; : ICommandWithResult&lt;IEnumerable&lt;TEntity&gt;&gt; { int Count(); } // used to create commands at runtime, looking up registered commands in an IoC container or service locator public interface ICommandFactory { TCommand Create&lt;TCommand&gt;() where TCommand : ICommand; } </code></pre> <p>Now I can create an interface to represent a specific command.</p> <pre><code>public interface IAccountsWithBalanceQuery : IQuery&lt;AccountWithBalance&gt; { Decimal MinimumBalance { get; set; } } </code></pre> <p>I can create a concrete implementation and use raw SQL, NHibernate HQL, whatever, and register it with my service locator.</p> <p>Now in my business logic I can do something like this:</p> <pre><code>var query = factory.Create&lt;IAccountsWithBalanceQuery&gt;(); query.MinimumBalance = 100.0; var overdueAccounts = query.Execute(); </code></pre> <p>You can also use a Specification pattern with <code>IQuery</code> to build meaningful, user-input-driven queries, rather than having an interface with million confusing properties, but that assumes you don't find the specification pattern confusing in its own right ;).</p> <p>One last piece of the puzzle is when your repository needs to do specific pre- and -post repository operation. Now, you can very easily create an implementation of your generic repository for a specific entity, then override the relevant method(s) and do what you need to do, and update your IoC or service locator registration and be done with it.</p> <p>However, sometimes this logic is cross-cutting and awkward to implement by overriding a repository method. So I created <code>IRepositoryBehavior</code>, which is basically an event sink. (Below is just a rough definition off the top of my head)</p> <pre><code>public interface IRepositoryBehavior { void OnAdding(CancellableBehaviorContext context); void OnAdd(BehaviorContext context); void OnGetting(CancellableBehaviorContext context); void OnGet(BehaviorContext context); void OnRemoving(CancellableBehaviorContext context); void OnRemove(BehaviorContext context); void OnFinding(CancellableBehaviorContext context); void OnFind(BehaviorContext context); bool AppliesToEntityType(Type entityType); } </code></pre> <p>Now, these behaviors can be anything. Auditing, security checking, soft-delete, enforcing domain constraints, validation, etc. I create a behavior, register it with the IoC or service locator, and modify my generic repository to take in a collection of registered <code>IRepositoryBehavior</code>s, and check each behavior against the current repository type and wrap the operation in the pre/post handlers for each applicable behavior.</p> <p>Here's an example soft-delete behavior (soft-delete means that when someone asks to delete an entity, we just mark it as deleted so it can't be returned again, but is never actually physically removed).</p> <pre><code>public SoftDeleteBehavior : IRepositoryBehavior { // omitted public bool AppliesToEntityType(Type entityType) { // check to see if type supports soft deleting return true; } public void OnRemoving(CancellableBehaviorContext context) { var entity = context.Entity as ISoftDeletable; entity.Deleted = true; // when the NHibernate session is flushed, the Deleted column will be updated context.Cancel = true; // set this to true to make sure the repository doesn't physically delete the entity. } } </code></pre> <p>Yes, this is basically a simplified and abstracted implementation of NHibernate's event listeners, but that's why I like it. A) I can unit test a behavior without bringing NHibernate into the picture B) I can use these behaviors outside of NHibernate (say the repository is client implementation that wraps REST service calls) C) NH's event listeners can be a real pain in the ass ;)</p>
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    1. COThanks for the snippets of code. I'll spend some time going over it more closely.
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    2. COI finally got more time to spend with this. I'm a little surprised by some of this code. You seemed to be saying that you like a very generic approach and yet you appear to be creating specialized interfaces, which is more specific than even the examples I had examined. Why is this needed? (BTW, if you ever feel like doing a more complete write up with source code, I'd love to publish something like this on my http://www.blackbeltcoder.com website.)
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