minor code rearange, quantity as optional parameter, removed some unused fields

src/ApplicationCore/Entities/OrderAggregate/Order.cs

@@ -33,11 +33,11 @@ namespace Microsoft.eShopWeb.ApplicationCore.Entities.OrderAggregate
         // but only through the method Order.AddOrderItem() which includes behavior.
         private readonly List<OrderItem> _orderItems = new List<OrderItem>();
 
-        public IReadOnlyCollection<OrderItem> OrderItems => _orderItems.AsReadOnly();
         // Using List<>.AsReadOnly() 
         // This will create a read only wrapper around the private list so is protected against "external updates".
         // It's much cheaper than .ToList() because it will not have to copy all items in a new collection. (Just one heap alloc for the wrapper instance)
         //https://msdn.microsoft.com/en-us/library/e78dcd75(v=vs.110).aspx 
+        public IReadOnlyCollection<OrderItem> OrderItems => _orderItems.AsReadOnly();
 
         public decimal Total()
         {

src/ApplicationCore/Interfaces/IBasketService.cs

@@ -7,7 +7,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
     {
         Task<int> GetBasketItemCountAsync(string userName);
         Task TransferBasketAsync(string anonymousId, string userName);
-        Task AddItemToBasket(int basketId, int catalogItemId, decimal price, int quantity);
+        Task AddItemToBasket(int basketId, int catalogItemId, decimal price, int quantity = 1);
         Task SetQuantities(int basketId, Dictionary<string, int> quantities);
         Task DeleteBasketAsync(int basketId);
     }

src/ApplicationCore/Services/BasketService.cs

@@ -20,7 +20,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Services
             _logger = logger;
         }
 
-        public async Task AddItemToBasket(int basketId, int catalogItemId, decimal price, int quantity)
+        public async Task AddItemToBasket(int basketId, int catalogItemId, decimal price, int quantity = 1)
         {
             var basket = await _basketRepository.GetByIdAsync(basketId);