About Collection

If I want repository return a group of data, which type should I choose? I saw some NHibernate DAO just return IList<T>, JP’s factory pattern returns IEnumerable, Rocky’s CSLA return costomized collection.

Which one is better? After I read this post about collection, I feel the power of the DDD, this post described a typical filter problem:

public class BookCatalog {
List<Book> findBooksByAuthor( … ) {..}
List<Book> findBooksByPublisher( … ) {..}
List<Book> findBooksByAuthorAndPublisher( … ) {..}

Can be easily solve by changing method return type from List<> to costomized collection:

public class BookCatalog {
BookCatalog findByAuthor( … ) {..}
BookCatalog findByPublisher( … ) {..}
BookCatalog findByPublishDates( DateTime,DateTime ) {..}
BookCatalog findByPublishingCountry( … ) {..}

The method chain is here:

BookCatalog.findByAuthor(“Frank”).findByPublisher(“Frank Inc.”)…

The new LinQ where syntax has the similar effect.

I still don’t know why IEnumeralbe should be used, seems it’s usefule when passing data to UI / controls, but why it’s also popular in passing data from DAL to upper layers.


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