Should I return IEnumerable<T> or IQueryable<T> from my DAL?

Question

I know this could be opinion, but I'm looking for best practices.

As I understand, IQueryable<T> implements IEnumerable<T>, so in my DAL, I currently have method signatures like the following:

IEnumerable<Product> GetProducts();
IEnumerable<Product> GetProductsByCategory(int cateogoryId);
Product GetProduct(int productId);

Should I be using IQueryable<T> here?

What are the pros and cons of either approach?

Note that I am planning on using the Repository pattern so I will have a class like so:

public class ProductRepository {

    DBDataContext db = new DBDataContext(<!-- connection string -->);

    public IEnumerable<Product> GetProductsNew(int daysOld) {
        return db.GetProducts()
          .Where(p => p.AddedDateTime > DateTime.Now.AddDays(-daysOld ));
    }
}

Should I change my IEnumerable<T> to IQueryable<T>? What advantages/disadvantages are there to one or the other?

Answer 1

It depends on what behavior you want.

• Returning an IList<T> tells the caller that they've received all of the data they've requested
• Returning an IEnumerable<T> tells the caller that they'll need to iterate over the result and it might be lazily loaded.
• Returning an IQueryable<T> tells the caller that the result is backed by a Linq provider that can handle certain classes of queries, putting the burden on the caller to form a performant query.

While the latter gives the caller a lot of flexibility (assuming your repository fully supports it), it's the hardest to test and, arguably, the least deterministic.