Comments on: Actually Querying with the Specification Pattern

By: Hosting company

Hosting company — Fri, 03 Feb 2012 09:59:00 +0000

Very good programming language….Its a valuable things..

By: hosting server

hosting server — Wed, 11 Jan 2012 09:11:00 +0000

Most of the people looking for this kind of valuable tips.

By: web hosting companies

web hosting companies — Tue, 10 Jan 2012 11:52:00 +0000

Your thoughts are also very good and i am very inspired from your post

By: hosting server

hosting server — Thu, 05 Jan 2012 10:31:00 +0000

Above the Query is excellent written skills.I enjoyed to visit this kind of valuable tips.

By: Paco

Paco — Wed, 16 Sep 2009 22:08:54 +0000

In that case, I would still go for the AreSatisfiedBy approach instead of the method in repository approach, because that decouples the specification form the type of collection to satisfy on. When the repository pattern is interpreted as “Mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects.” the repository should be an IEnumerable and not have a method that returns an IEnumerable

public interface IRepository : IEnumerable
{
}

The specification can be used as
IEnumerable result = new MySpecification().AreSatisfiedBy(repository)

instead of IEnumerable result = repository.Get(new MySpecification());

there is nothing wrong with a Get method in the repository that has a specification as argument, but with the “repository is a collection” approach, the repository interface can be substituded by any other collection interface in both tests and the real code.

By: Chris Missal

Chris Missal — Wed, 16 Sep 2009 19:08:26 +0000

@Paco

When I ask a repository for something, I’m not concerned whether it’s getting me data from an in memory collection or a database. I just want to know that it holds what I’m looking for, whatever the implementation may be.

By: Paco

Paco — Wed, 16 Sep 2009 18:44:43 +0000

“this was set up for in memory collections” -> Why do you call it repository? It’s a confusing term to use for an in-memory collection.

Why don’t you put the list option in separate repository classes instead of in the specification?
ISpecification
{
bool IsSatisfiedBy(T entity);
IEnumerable AreSatisfiedBy(IEnumerable entities);
}

By: Chris Missal

Chris Missal — Wed, 16 Sep 2009 14:52:44 +0000

@Ryan
Testing or other, this was set up for in memory collections. When making any kind of remote call, you’re right. I suppose you could argue that showing code like this could lead to those bad practices of pulling an entire data set (or using Linq to SQL) ;)

@Jeremy

Good point, I believe you’re correct. I only used the yield to take advantage of deferred execution.

By: Jeremy Wiebe

Jeremy Wiebe — Wed, 16 Sep 2009 14:35:29 +0000

Great examples, Chris.

I’m wondering though, why do you need the ‘foreach’ in the last example? Couldn’t you just return:

return GetAllEntities().Where(specification.IsSatisfiedBy);

Does the yield buy you anything that I’m missing?

By: Ryan Riley

Ryan Riley — Wed, 16 Sep 2009 14:28:51 +0000

While the abstract repository implementation works well for testing, I hope everyone will keep in mind that pulling back an entire table in something like LINQ to SQL will cause a large performance hit. If you would like to see an example of specifications used in filtering results from the database, check out Linq, the Specification Pattern and Encapsulation (code).