2014年1月8日星期三

entity framework 4.1 - Should a Repository return IEnumerable , IQueryable or List? - Stack Overflow

up vote 3 down vote favorite
1
I'd like to make my application as flexible as possible, but not dig myself into a hole by making my Interface too specific.
What is the best object type for a repository? IEnumerable, IQueryable, or List?
The technologies I'm considering using are
  • Azure App Fabric Caching
  • Entity Framework 4.1
  • Possibly Windows Server AppFabric

3 Answers
up vote 3 down vote accepted
I would say build your DAL using IQueryable, and pass it around, make sure your object contects lifetime is the request. This way you will get benefit of delayed execution, but are exposed to the risk of inefficient querying of database.
Then make sure you performance test your application( or atleast the parts that are most likely to get traffic) and see the dataaccess patterns. Create specialized methods in your DAL to retreive fully materialized onjects and make these queries as pre compiled queries.
a sample of the repository interface would be like
  public interface IDataContext
  {
        void Add<T>(T entity) where T : BaseEntity;
        void Delete<T>(T entity) where T : BaseEntity;
        IQueryable<T> Find<T>(Expression<Func<T, bool>> where) where T : BaseEntity;
   }
where BaseEntity is the base class to all our classes, it looks like, this class is not mapped to any table in db
public abstract class BaseEntity
{
        public int Id { get; set; }
        public DateTime CreateDateTime { get; set; }
        public string CreateUser { get; set; }
        public DateTime ModDateTime { get; set; }
        public string ModUser { get; set; }
        public byte[] RowVersion { get; set; }
}
Expression<Func<T, bool>> would pass the whole expression to your repository instead of just Func, since EF works on expression to generate sql query, a typical use would be
ICollection<WFGroup> wgGroups = this.dataContext.Find<WFGroup>((w) => true).ToList();
where WFGroup is a class derived from BaseEntity, I typically use lazy loading and proxy and dont detach/attach objects to a context.

I'm using a caching service that requires a materialized query. IEnumerable<T> is the best common denominator I can find. Does this mean I have a layer on top of my IQueryable<T> DAL that flattens everything to IEunmerable<T>? If so what is <T>? Entity model, view, or viewmodel? – makerofthings7 Nov 17 '11 at 15:57
IEnumerable is no guarantee of materialized object IQueryable is also IEnumerable. T in my case is Entity,we have an interface IDataContext that the repository implements and every method is generic T where T is constrained to our base entity. – np-hard Nov 17 '11 at 16:04
Interesting, I see how it can work for a 1:1 table to entity mapping, but I'm curious how you handle more complex things like 1:Many relationships... stackoverflow.com/q/8095728/328397 – makerofthings7 Nov 17 '11 at 16:39
i updated the answer with a sample interface, 1:n is handled in the entity model. This was a little hard to implement in EF 4 with a modified t4 but with EF 4.2 its a breeze – np-hard Nov 17 '11 at 17:12
Thanks, that looks like the direction I should go. I've only read about Expression<Func<T, bool>> but not sure how to use it. Can you add one more sample? Also, just so I understand BaseEntity could be something like Students or EnrolledClasses right? – makerofthings7 Nov 17 '11 at 17:31
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up vote 2 down vote
It depends on whether you wish to have any future queries performed on the entity and whether these should be in memory or not:
  • If there are to be future queries and the db should do the work return IQueryable
  • If there are to be future queries and it is to be done in memory return IEnuerable
  • If there are to be no further queries and all the data will need to be read return an IList, ICollection etc


Where is it OK to issue queries based on this past the DAL? Should I pass IQueryable to the controller? The View? – makerofthings7 Nov 12 '11 at 2:46
1
You can still cause queries to run in the DB if you use IEnumerable – the documentation for IQueryable says it's intended for implementing query providers, not to make this distinction. I prefer not using IEnumerable once querying the database is finished, because using a collection type makes sure you're returning materialised results and not the unevaluated query by mistake. – millimoose Nov 12 '11 at 2:54
1
This sort of question either relates to performance tuning, in which case performance should be measured with all considered options (and the most performant chosen) or down to architectural considerations. In the case of the latter I'd suggest exposing an IQueryable to the controller is acceptable (since controller knowledge of domain objects is okay), but the view / view model should be dumb and not have knowledge of such things... But this is only my opinion! – rich.okelly Nov 12 '11 at 2:55

@makerofthings7: In a strict layered architecture, it's not OK to issue DB-backed queries anywhere outside the DAL. It's OK to do in-memory queries to transform data structures anywhere and you can't prevent it anyway since all collections can be queried in memory. – millimoose Nov 12 '11 at 2:56
up vote 2 down vote
How likely are you to ever need to return a custom implementation of IEnumerable (not a collection) from your DAL? (To answer this question, look at your previous projects and count how many of those or of yield returns you have around.)
If the answer is "not very", I'd just return ICollection or even arrays (if you want to prevent the query results from being inadvertently modified.) In a pinch, if you ever need to change a query to "stream" results with a custom IEnumerable, you can always have the old method call the new one and materialise the results to keep compatibility with older clients.
answered Nov 12 '11 at 3:04
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