Cleaning things up and getting add item to cart working for anonymous or authenticated users.

src/ApplicationCore/Entities/Basket.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Entities
 {
@@ -6,5 +7,21 @@ namespace Microsoft.eShopWeb.ApplicationCore.Entities
     {
         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;
+        }
     }
 }