Changed CartItem.CartId type from string to Guid. Needed this change to ease the creation of an index on the CartId column in the database. String is not an indexable type, guid (uniqueidentifier in SQL Server) is. Adding that index has a big impact on database performance for the MusicStore app.

src/MusicStore/Models/CartItem.cs

@@ -9,7 +9,7 @@ namespace MusicStore.Models
         public int CartItemId { get; set; }
 
         [Required]
-        public string CartId { get; set; }
+        public Guid CartId { get; set; }
         public int AlbumId { get; set; }
         public int Count { get; set; }
 

src/MusicStore/Models/ShoppingCart.cs

@@ -10,9 +10,9 @@ namespace MusicStore.Models
     public class ShoppingCart
     {
         private readonly MusicStoreContext _dbContext;
-        private readonly string _shoppingCartId;
+        private readonly Guid _shoppingCartId;
 
-        private ShoppingCart(MusicStoreContext dbContext, string id)
+        private ShoppingCart(MusicStoreContext dbContext, Guid id)
         {
             _dbContext = dbContext;
             _shoppingCartId = id;
@@ -21,7 +21,7 @@ namespace MusicStore.Models
         public static ShoppingCart GetCart(MusicStoreContext db, HttpContext context) 
             => GetCart(db, GetCartId(context));
 
-        public static ShoppingCart GetCart(MusicStoreContext db, string cartId)
+        public static ShoppingCart GetCart(MusicStoreContext db, Guid cartId)
             => new ShoppingCart(db, cartId);
 
         public async Task AddToCart(Album album)
@@ -166,17 +166,22 @@ namespace MusicStore.Models
         }
 
         // We're using HttpContextBase to allow access to sessions.
-        private static string GetCartId(HttpContext context)
+        private static Guid GetCartId(HttpContext context)
         {
-            var cartId = context.Session.GetString("Session");
+            Guid cartId;
+            string cartIdString = context.Session.GetString("Session");
 
-            if (cartId == null)
+            if (cartIdString == null)
             {
                 //A GUID to hold the cartId. 
-                cartId = Guid.NewGuid().ToString();
+                cartId = Guid.NewGuid();
 
                 // Send cart Id as a cookie to the client.
-                context.Session.SetString("Session", cartId);
+                context.Session.SetString("Session", cartId.ToString());
+            }
+            else
+            {
+                cartId = Guid.Parse(cartIdString);
             }
 
             return cartId;