Am I using IRepository correctly?

Question

I'm looking to use the IRepository pattern (backed by NHibernate, if it matters) in a small project. The domain is a simple one, intentionally so to allow me to focus on understanding the IRepository pattern. The lone domain class is Movie, with properties for Year, Genre, and Title. My intent would be to "get" movies whose properties match criteria of the aforementioned types.

Convention seems to be to have a generic IRepository interface, similar to the following:

public interface IRepository<T>
{
    T Get(int id);
    T[] GetAll();
    void Add(T item);
    void Update(T item);
    void Delete(T item);
}

With a base implementation:

public abstract class Repository<T> : IRepository<T>
{
    public T Get(int id) { ... }
    public T[] GetAll() { ... }
    public void Add(T item) { ... }
    public void Update(T item) { ... }
    public void Delete(T item) { ... }
}

Then to have a domain-specific interface:

public interface IMovieRepository
{
    Movie[] GetByGenre(Genre genre);
    Movie[] GetByYear(int year);
    Movie[] GetByTitle(string title);
}

With an implementation that also extends the base Repository class:

public class MovieRepository : Repository<Movie>, IMovieRepository
{
    public Movie[] GetByGenre(Genre genre) { ... }
    public Movie[] GetByYear(int year) { ... }
    public Movie[] GetByTitle(string title) { ... }
}

I would need to add necessary implementation to the base class as well as the concrete one, using NHibernate, but I would like to know if I am on the right track with this setup.

There seems to be a fair bit of overhead for just one domain class, though it would be less noticeable if there were multiple domain classes involved. Right now I'm trying to keep it simple so I can pin down the concept.

Answer 1

1. try not to pass back an array. use IEnumerable<T>, ICollection<T> or IList<T>, this will loosely couple your code further.

2. your IMovieRepository interface. this repository includes the CRUD. therefore make it

IMovieRepository : IRepository<Movie> {}

This will not change your MovieRepository class as that will implement the interface correctly. it will allow you to decouple your classes if you wish to change the implementation at a later date.

finally. this is fine for one of the methods. as you have specialised functionality you have specialised the repository to suit.

there are other ways, which enable you to use 1 repositry class and pass in the required query. This is called the Specification pattern. I did a project which uses this located on codeplex with report http://whiteboardchat.codeplex.com

the other way would to to have a method to pass in the criteria. there is a open source project called Sharp Architecture, which i believe has this coded up.

Hope this helps