# IT:AD:Patterns:Specification Pattern #



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<panel title="Summary">

Used extensively in [[IT/AD/Patterns/DDD/Components/App.Domain.Repositories/]].


It's separating  the  decision  as  to  which object types should be selected in a query from the object that makes the selection.  
Writing code like this allows reusing a  Domain Layer Specification within a repository as a method for querying:

    IEnumerable<Customer> customers = customerRepository.GetBySpec(IsPremiumCustomerSpecification);

That said, the original MF pattern was written prior to LINQ.


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## Considerations ##



* Pros:
    * End user can built up and save precompiled Queries. 
        * "Select all (Invoices with Total > 300.00) AND (Country = GB)"
    * We decouple the design of requirements, compliance and validation. 
        * ??? EXAMPLE ???
    * It allows definition of clear and declarative queries
        * Instead of Linq all over the Application Layer, untested until run, Queries can be tested in isolation, and reused.

## Recipe ##

[TODO]

## Resources ##
* http://blog.willbeattie.net/2011/02/specification-pattern-entity-framework.html
* http://nblumhardt.com/archives/implementing-the-specification-pattern-via-linq/
* http://guildsocial.web703.discountasp.net/dasblogce/2010/02/20/SpecificationPatternAndLinq.aspx
* http://linqspecs.codeplex.com/