Specific generic interfaces

Question

I'm refactoring all my repository interfaces of various types. Most of them contain very similar methods like Add, Update but some have methods which only makes sense for a specific type. This is a best practices question.

I thought about using generics to straighten things up.

 public interface IRepository<T>
 {
      T Get(int id);
      void Add(T x);
 }

But now for the specific methods. I could ofcourse "subclass" the interface, but then I'm not better off than before. I would have code like:

 IUserRepository<User> users;

One neat way would be if I could have multiple constraints like:

 public partial interface IRepository<T>
 {
      T Get(int id);
      void Add(T x);
 }

 public partial interface IRepository<T> where T: User
 {
      T Get(Guid id);
 }

 public partial interface IRepository<T> where T: Order
 {
      T Get(string hash);
 }

But the compiler complains about conflicting inheritance. Annother way would be contraints on the methods:

 public partial interface IRepository<T>
 {
      T Get(int id);
      void Add(T x);

      T Get(Guid id) where T: User;
      T Get(string hash) where T: Order;
 }

But that's not quite the way these this work is it. Compiler fathoms not my intentions and wants a type definition on the method, of course.

Right now I just have methods that throw NotImplemented. Ugly.

I'm looking for a solution that will make me kick myself.

Answer 1

public interface IRepository<TEntity, TId>
 {
      TEntity Get(TId id);
      void Add(T x);
 }

public class UserRepository : IRepository<User, Guid>
{
    public User Get( Guid id ) 
    {
        // ...
    }

    public void Add( User entity) 
    {
        // ...
    }
}

public class OrderRepository : IRepository<Order, string> 
{
    //...
}

Answer 2

Here are my thoughts on a similar question:

http://stackoverflow.com/questions/1230571/advantage-of-creating-a-generic-repository-vs-specific-repository-for-each-objec/1231473#1231473

The gist is that domains often can't be generalized, and another approach is in order. I give an example of using domain-specific interfaces but with a generalized base class.