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    <p>You should not have other repository interfaces besides your generic <code>IRepository&lt;T&gt;</code>. If you need those, you are missing an abstraction.</p> <p>For instance, a common reason for people to have custom repository interfaces is because they have a custom query that some repository has, while other don't. For instance:</p> <pre><code>public interface IEmployeeRepository : IRepository&lt;Employee&gt; { Employee GetEmployeeOfTheMonth(int month); } </code></pre> <p>The problem here is that the <code>IEmployeeRepository</code> is abused for a 'custom query'. Custom queries deserve their own (generic) abstraction:</p> <pre><code>// Defines a query public interface IQuery&lt;TResult&gt; { } // Defines the handler that will execute queries public interface IQueryHandler&lt;TQuery, TResult&gt; where TQuery : IQuery&lt;TResult&gt; { TResult Handle(TQuery query); } </code></pre> <p>With this abstraction we can add custom queries to the system, without the need of creating <code>IRepository&lt;T&gt;</code> derivatives:</p> <pre><code>public class GetEmployeeOfTheMonthQuery : IQuery&lt;Employee&gt; { [Range(1, 12)] public int Month { get; set; } } class GetEmployeeOfTheMonthHandler : IQueryHandler&lt;GetEmployeeOfTheMonthQuery, Employee&gt; { public Employee Handle(GetEmployeeOfTheMonthQuery query) { // todo: query the database, web service, disk, what ever. } } </code></pre> <p>A consumer that needs to know the employee of the month, can now simply take a dependency on <code>IQueryHandler&lt;GetEmployeeOfTheMonthQuery, Employee&gt;</code> and execute the query as follows:</p> <pre><code>var query = new GetEmployeeOfTheMonthQuery { Month = 11 }; var employee = this.employeeOfMonthHandler.Handle(query); </code></pre> <p>This might seem like overhead, but this model is very flexible, scalable, and has many interesting benefits. For instance, it is very easy to add cross-cutting concerns by wrapping handlers with decorators.</p> <p>This also allows our reposities to be hidden behind one generic interface, which allows us to easily batch register them at once and add decorators to them as well.</p> <p>For more in depth information, read this article: <a href="https://cuttingedge.it/blogs/steven/pivot/entry.php?id=92" rel="nofollow noreferrer">Meanwhile… on the query side of my architecture</a>.</p>
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    1. COvery well explained
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