Question

I have a standard repository interface in C# which includes the following methods:

IEnumerable<T> GetAll();
T GetById(int id);
void Delete(T entity);
void Add(T entity);

At my domain layer all I am instantiating is a new Unit of Work wrapper and passing it to a repository. The Unit of Work wrapper class hides whether I’m using NHibernate or Entity Framework and exposes a Commit() method.

In my domain layer, how can I query for objects which meet only a particular criteria?

I think what I’m doing at the moment is terribly inefficient. I am currently doing this:

var results = myRepository.GetAll().Where......

If I have a very large amount of objects, is GetAll() going to return every one of them before filtering out the ones I don’t need? How can I prevent unwanted object from being returned at all?

Obviously I could add more methods to the interface, but this doesn’t seem in keeping with exposing only CRUD operations via the interface.

i.e. – I don’t think I should be adding things like (but maybe I’m wrong):

IList<T> GetAllWhereMeetsMyCriteria();

Answer 1

Have a look at the repository pattern used in this blog post [weblogs.asp.net]. I have found the source code to employ a few interesting patterns that I keep returning back to and checking again. The code implements both repository and unit of work patterns.

To answer your specific question, the repository interface contains the method:

IEnumerable<T> GetMany(Expression<Func<T, bool>> where);    

It is implemented as follows in the RepositoryBase<T> class:

public virtual IEnumerable<T> GetMany(Expression<Func<T, bool>> where)
{
    return dbset.Where(where).ToList();
}

Probably both EF and NHibarnate will have the interfaces necessary to support this usage.