eShopOnWeb/src/ApplicationCore/Entities/Basket.cs

using System.Collections.Generic;
using System.Linq;

namespace Microsoft.eShopWeb.ApplicationCore.Entities
{
    public class Basket : BaseEntity<string>
    {
        public string BuyerId { get; set; }
        public List<BasketItem> Items { get; set; } = new List<BasketItem>();

        public void AddItem(int productId, decimal unitPrice, int quantity = 1)
        {
            if(!Items.Any(i => i.ProductId == productId))
            {
                Items.Add(new BasketItem()
                {
                    ProductId = productId,
                    Quantity = quantity,
                    UnitPrice = unitPrice
                });
                return;
            }
            var existingItem = Items.FirstOrDefault(i => i.ProductId == productId);
            existingItem.Quantity += quantity;
        }
    }
}