Question

Here is the simple snippet:

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

  public class Repository<T> : IRepository<T> where T : class 
  {
    private readonly IRepositoryContext _repositoryContext;
    private IObjectSet<T> _objectSet;

  public Repository(IRepositoryContext repositoryContext)
  {
    _repositoryContext = repositoryContext; 
    _objectSet = repositoryContext.GetObjectSet<T>();    
  }

   public virtual T Get(int id)
   {
       return ObjectSet.First(x=> x.Id == id) 
      // that wouldn't work because there is no guarantee that T entity has Id property
   }

Now, as you can see I can instantiate Repository for any entity object and use methods defined (although in the example we have only Get(). But I can’t use any constrains in expressions, unless I create non-abstract classes for each entity of T based on IRepository<T> and then implement methods the way I want.

But what if I need to use a method like Get which implementation stays the same for probably all entities (every entity has an Id).

How to do that?

I thought, I could create an abstract entity in EF data model designer, mapped to nothing, and mark that as the base type for all other entities, but It’s asking for a mapping i.e. table. I tried to go with a complex type instead – that wouldn’t let me inherit from it.

So, show me please the most efficient way to achieve that

Answer 1

That’s not really the repository’s role to know whether there’s an ID or not in your object.

Most repositories implement the Single method like this

public T Single( Expression<Func<T, bool>> predicate ) {
    return ObjectSet.Single<T>( predicate );
}