C# generics is keyword covariance / contravariance type inferral

Question

Please consider this interface:

public interface IInitialiazableEntity<TRepository> where TRepository : class, IRepository
{
    void Initialize(TRepository repository);
}

This class (snippet):

public class SomeFacade<TRepository> where TRepository : class, IRepository
{
        public void Add<TEntity>(TEntity entity) where TEntity : AbstractEntity
        {
            try
            {
                if (entity is IInitialiazableEntity<TRepository>)
                    (entity as IInitialiazableEntity<TRepository>).Initialize(this.Repository);
            }
            catch (Exception ex)
            {
                this.Logger.Error(ex);
            }
        }
}

And the entity:

public class Student : AbstractEntity, IInitialiazableEntity<IRepository>
{ 
    void Initialize(IRepository repository) { ... }
}

Since the student is only IInitialiazableEntity<IRepository> and the facade will have an actual repository which is more specialized than the basic IRepository (i.e. it will be IMySpecialRepository : IRepository), will the is keyword realize that it can cast the IMySpecialRepository and pass it to the Initialize method of the entity? Or if not, how to do it?

Have you tried it? Also, what are IMySpecialRepository and IMySpecializedRepository? — ken2k, Nov 03 '14 at 13:42
I am sorry, I know I can test this, and then try find a solution, but I'm in an extra hurry. And I think It will be useful for other people (given the title/tags are right). — h.alex, Nov 03 '14 at 13:45
@ken2k the special(ized) repository (there was a typo, both are the same, I corrected it) - is anything inheriting from the base IRepository. — h.alex, Nov 03 '14 at 13:47
Sure you can do it. The thing is, should you? The actual type should be none of your concerns. — Erti-Chris Eelmaa, Nov 03 '14 at 13:51

score 1 · Answer 1 · answered Nov 03 '14 at 13:46

1

Assuming your entity is of type Student, the 'is' will return false; this is because Student doesn't implement the more specific interface specialization. You don't need to cast to this more specific interface, though. This should work just fine:

public class SomeFacade<TRepository> where TRepository : class, IRepository
{
        public void Add<TEntity>(TEntity entity) where TEntity : AbstractEntity
        {
            try
            {
                if (entity is IInitialiazableEntity<IRepository>)
                    (entity as IInitialiazableEntity<IRepository>).Initialize(this.Repository);
            }
            catch (Exception ex)
            {
                this.Logger.Error(ex);
            }
        }
}

answered Nov 03 '14 at 13:46

Dan Bryant

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But the Student entity might use the basic IRepository, while some other entity might implement IInitialiazableEntity with a more specialized repository, thus I need to support it. – h.alex Nov 03 '14 at 13:49
But the top of the inheritance will be the TRepository that the facade has. No entity will use a more specialized repo than the facade. Or I can just make the entities have to implement the initializable with the most specialized repo, and be done with it, then the is will return true.. what do you think? – h.alex Nov 03 '14 at 13:51

score 1 · Accepted Answer · answered Nov 03 '14 at 13:50

Currently, if you had an instance SomeFacade<IMySpecialRepository> the code will not work.

You have two options, one is the answer provided by Dan Bryant, the other is to make IInitialiazableEntity<TRepository> contravariant.

Your interface declaration will become (note the in on the generic type):

public interface IInitialiazableEntity<in TRepository> where TRepository : class, IRepository
{
    void Initialize(TRepository repository);
}

This will allow your is check and cast to work, as IInitialiazableEntity<IRepository> can be cast to IInitialiazableEntity<IMySpecialRepository> when using a contravariant generic.

See here for more information about Covariance and Contravariance in generics.

Thank you Sir, that one in makes all the difference. I never would've went this way hadn't I asked. Thanks for sharing. — h.alex, Nov 03 '14 at 14:10

C# generics is keyword covariance / contravariance type inferral

2 Answers2