public class Basket : EventSource
{
    private int Id;

    private List<Item> Items = new List<Item>();

    public Basket()
    {
        On<ItemAddedToBasket>(e => {
            var item = new Item(@event =>
            {
                switch(@event)
                {
                    case ItemCountIncreased increased:
                        increased.BasketId = Id;
                }
                Apply(@event);
            });
            item.Play(e);
            Items.Add(item);
        });

        On<ItemCountIncreased>(e => {
            var item = Items.Single(candidate => candidate.Id == e.ItemId);
            item.Play(e);
        })
    }

    public void AddItem(int item)
    {
        Apply(new ItemAddedToBasket {
            BasketId = Id,
            ItemId = item
        });
    }
    
    //behavior - v1
    public void IncreaseItemCount(int item)
    {
        var item = Items.Single(candidate => candidate.Id == e.ItemId);
        item.IncreaseCount();
    }
    //behavior - v2 
    public Item FindItem(int item)
    {
        return Items.Single(candidate => candidate.Id == e.ItemId);
    }

    public class Item : Entity
    {
        public int Id { get; }

        private int Count;

        public Item(Action<object> applier) 
            : base(applier)
        {
            On<ItemAddedToBasket>(e => Id = e.ItemId);
            On<ItemCountIncreased>(e => Count++);
        }

        public void IncreaseCount()
        {
            Apply(new ItemCountIncreased { ItemId = Id });
        }
    }
}


//Handle AddItemToBasket
var basket = baskets.FindById(msg.Body.BasketId);
basket.AddItem(msg.Body.ItemId);

//Handle IncreaseItemCountInBasket
var basket = baskets.FindById(msg.Body.BasketId);
basket.IncreaseItemCount(msg.Body.ItemId);

or

var basket = baskets.FindById(msg.Body.BasketId);
var item = basket.FindItem(msg.Body.ItemId);
item.IncreaseCount();

BaskCreated
ItemAddedToBasket
ItemCountIncreased