Nsryjdtyk

Playing around with genericity, I built a simple social network model, base on persons and relations between them.

All of those are stored in a container.

For each container, I want to be able to add some validation to check some specific constraints.

Here is the maybe overly complex model, but as said, I'm playing with genericity, so it's kind of on purpose:

public interface IContainer<TPerson, TRelation>

    where TRelation : IRelation<TPerson>

{

    IReadOnlyCollection<TPerson> Persons { get; }

    IReadOnlyCollection<TRelation> Relations { get; }

}



public class DefaultContainer<TPerson, TRelation>

    : IContainer<TPerson, TRelation>

    where TRelation : IRelation<TPerson>

{

    private readonly HashSet<TPerson> _persons;

    private readonly List<TRelation> _relations;



    //...

}



public class DummyValidator<TContainer, TPerson, TRelation>

    : IValidator<TContainer, TPerson, TRelation>

    where TContainer : IMutableContainer<TPerson, TRelation>

    where TRelation : IRelation<TPerson>

{

    public bool CanAddRelations(TContainer container, IEnumerable<TRelation> relations)

    {

        // guard clauses



        return true; // whatever

    }



    public bool CanCreate(IEnumerable<TPerson> persons, IEnumerable<TRelation> relations)

    {

        // guard clauses



        return CanAddRelations(new DefaultContainer<TPerson, TRelation>(), relations);

    }



    //...

}



[Fact]

public void Test()

{

    var sut = new DummyValidator<IContainer<int, IRelation<int>>, int, IRelation<int>>();

    sut.CanCreate(Enumerable.Empty<int>(), Enumerable.Empty<IRelation<int>>());

}

Take a look at the CanCreate method of the DummyValidator class.
It creates a new container (implementing TContainer):

return CanAddRelations(new DefaultContainer<TPerson, TRelation>(), relations);

So everything should be fine, as far as I can tell. But the compiler complains, saying:

cannot convert from 'Alcuin.Graph.Containers.DefaultContainer' to 'TContainer'

My question is: why does it complain about that, even if every generic parameter is described and always equivalent ?

So I tried some explicit casts to investigate:

return CanAddRelations((TContainer)new DefaultContainer<TPerson, TRelation>(), relations);

and

return CanAddRelations((IMutableContainer<TPerson, TRelation>)new DefaultContainer<TPerson, TRelation>(), relations);

both fail, but:

return CanAddRelations((TContainer)(IMutableContainer<TPerson, TRelation>)new DefaultContainer<TPerson, TRelation>(), relations);

compiles and runs correctly...

Can anyone shed some light on what is happening here ?
Am I missing something obvious ? Am I reaching the limits of C#'s genericity engine ?