Partial refactoring about Order and OrderItems.

Set the fields to explicit private
Using .ToReadOnly(), but still might need to use List<> instead HashSet<> so we won't need the .ToList() conversion which is "expensive"..

src/Services/Ordering/Ordering.Domain/AggregatesModel/BuyerAggregate/Buyer.cs

@@ -12,7 +12,7 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.B
 
         private HashSet<PaymentMethod> _paymentMethods;
 
-        public IEnumerable<PaymentMethod> PaymentMethods => _paymentMethods?.ToList().AsEnumerable();
+        public IEnumerable<PaymentMethod> PaymentMethods => _paymentMethods?.ToList().AsReadOnly();
 
         protected Buyer() { }
 

src/Services/Ordering/Ordering.Domain/AggregatesModel/OrderAggregate/Order.cs

@@ -9,24 +9,41 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.O
     public class Order
         : Entity
     {
+        // DDD Patterns comment
+        // Using private fields, allowed since EF Core 1.1, is a much better encapsulation
+        // aligned with DDD Aggregates and Domain Entities (Instead of properties and property collections)
+        private DateTime _orderDate;
+
+        //TO DO: These fields need to be converted to a VALUE-OBJECT "Address" 
         private string _street;
         private string _city;
         private string _state;
         private string _country;
         private string _zipCode;
-        private DateTime _orderDate;
 
         public Buyer Buyer { get; private set; }
-        int _buyerId;
+        private int _buyerId;
 
         public OrderStatus OrderStatus { get; private set; }
-        int _orderStatusId;
+        private int _orderStatusId;
 
-        HashSet<OrderItem> _orderItems;
-        public IEnumerable<OrderItem> OrderItems => _orderItems.ToList().AsEnumerable();
+        // DDD Patterns comment
+        // Using a private collection field, better for DDD Aggregate's encapsulation
+        // so OrderItems cannot be added from "outside the AggregateRoot" directly to the collection,
+        // but only through the method OrderAggrergateRoot.AddOrderItem() which includes behaviour.
+        private readonly HashSet<OrderItem> _orderItems;
+
+        //TODO: Use List<> instead of HashSet<> because of the comment below
+        // So we won't need the ".ToList()"
+
+        public IEnumerable<OrderItem> OrderItems => _orderItems.ToList().AsReadOnly();
+        // Using List<>.AsReadOnly() 
+        //This will create a read only wrapper around the private list.
+        //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 PaymentMethod PaymentMethod { get; private set; }
-        int _paymentMethodId;
+        private int _paymentMethodId;
 
         protected Order() { }
 
@@ -46,7 +63,10 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.O
             _orderItems = new HashSet<OrderItem>();
         }
 
-
+        // DDD Patterns comment
+        // This Order AggregateRoot's method "AddOrderitem()" should be the only way to add Items to the Order,
+        // so any behavior (discounts, etc.) and validations are controlled by the AggregateRoot 
+        // in order to maintain consistency between the whole Aggregate. 
         public void AddOrderItem(int productId, string productName, decimal unitPrice, decimal discount, string pictureUrl, int units = 1)
         {
             var existingOrderForProduct = _orderItems.Where(o => o.ProductId == productId)

src/Services/Ordering/Ordering.Domain/AggregatesModel/OrderAggregate/OrderItem.cs

@@ -6,6 +6,9 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.O
     public class OrderItem
         : Entity
     {
+        // DDD Patterns comment
+        // Using private fields, allowed since EF Core 1.1, is a much better encapsulation
+        // aligned with DDD Aggregates and Domain Entities (Instead of properties and property collections)
         private string  _productName;
         private string  _pictureUrl;
         private int     _orderId;
@@ -15,7 +18,6 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.O
 
         public int ProductId { get; private set; }
 
-
         protected OrderItem() { }
 
         public OrderItem(int productId, string productName, decimal unitPrice, decimal discount, string PictureUrl, int units = 1)

src/Services/Ordering/Ordering.Infrastructure/OrderingContext.cs

@@ -119,7 +119,8 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Infrastructure
             orderConfiguration.Property<int>("PaymentMethodId").IsRequired();
 
             var navigation = orderConfiguration.Metadata.FindNavigation(nameof(Order.OrderItems));
-
+            // DDD Patterns comment:
+            //Set as Field (New since EF 1.1) to access the OrderItem collection property through its field
             navigation.SetPropertyAccessMode(PropertyAccessMode.Field);
 
             orderConfiguration.HasOne(o => o.PaymentMethod)