Question

I have a list:

private readonly IList<IList<GameObjectController>> removeTargets;
private readonly IList<IList<GameObjectController>> addTargets;

PickUp inherits from GameObjectController. But when I try this:

public IList<PickUp> Inventory

// ... 

gameObjectManager.MoveFromListToWorld(this, user.Model.Inventory);

// ...

    // This queues everything up to be removed, until ProcessMoves...() is called
    public void MoveFromWorldToList(GameObjectController removedFromWorld, IList<GameObjectController> addTarget)
    {
        toBeRemoved.Add(removedFromWorld);
        addTargets.Add(addTarget);
    }

// ...

    /// <summary>
    /// Removes all the GameObjects on which removal is requested from the world.
    /// </summary>
    public void ProcessMovesFromListToWorld()
    {
        for (int i = 0; i < toBeAdded.Count; i++)
        {
            GameObjectController moved = toBeAdded[i];
            addGameObjectToWorld(moved);

            if (removeTargets[i] != null)
            {
                removeTargets[i].Remove(moved);
            }
        }
        toBeAdded.Clear();
        removeTargets.Clear();
    }

I get a compiler error:

cannot convert from
‘System.Collections.Generic.IList<PickUp>’
to
‘System.Collections.Generic.IList<GameObjectController>’

Why does this occur? Shouldn’t this be fine, since PickUp is a subclass of GameObjectController? Do I need something like Java’s Map<E extends GameObjectController>?

Earlier, I was having a similar problem, where I was trying to implicitly cast inventory from an IList to an ICollection. Is this the same problem?

Answer 1

See my other answer here:
How to return an IQueryable<Something> as an IQueryable<ISomething>

Short version:

Remember that IList<PickUp> and IList<GameObjectController> are two (and only two) completely different classes in the type system. The only inheritance relationship they have comes from the System.Collections branch of the inheritance tree; there is none at all on the GameObjectController branch of the inheritance tree. You might as well try to cast an IList<double> to an IList<ArgumentNullException>.

What you can do instead is call the Cast<T>() extension method that is available for IEnumerable<T>.