Basket.cs

public class Basket : BaseEntity, IAggregateRoot
{
    private readonly List<BasketItem> _items = new List<BasketItem>();
    public string BuyerId { get; private set; }
    public IReadOnlyCollection<BasketItem> Items => _items.AsReadOnly();
    public int TotalItems => _items.Sum(i => i.Quantity);

    public Basket(string buyerId)
    {
        BuyerId = buyerId;
    }
    
    // Rest of the code omitted
}