e.g. 5.12 Implement and Interface in ShoppingCart

e.g. 5.13 An Extension Method That works on an Interface

chap5_essential_language_features/essential_csharp_features/Program.cs

@@ -30,10 +30,10 @@ namespace automatically_implemented_properties
 
     public static class MyExtensionMethod
     {
-        public static decimal TotalPrices(this ShoppingCart cartParam)
+        public static decimal TotalPrices(this IEnumerable<Product> productEnum)
         {
             decimal total = 0;
-            foreach (Product prod in cartParam.Products)
+            foreach (Product prod in productEnum)
             {
                 total += prod.Price;
             }

chap5_essential_language_features/essential_csharp_features/ShoppingCart.cs

@@ -1,9 +1,20 @@
-using System.Collections.Generic;
+using System.Collections;
+using System.Collections.Generic;
 
 namespace automatically_implemented_properties
 {
-    public class ShoppingCart
+    public class ShoppingCart : IEnumerable<Product>
     {
         public List<Product> Products { get; set; }
+
+        public IEnumerator<Product> GetEnumerator()
+        {
+            return Products.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
     }
 }