3.4.1 使用过滤式扩展方法（P43-44）

 public static IEnumerable<Product> FilterByCategory(this IEnumerable<Product>productEnum,string categoryParam)
        {
            foreach (Product prod in productEnum)
            {
                if (prod.Category == categoryParam)
                {
                    yield return prod;
                }
            }

        }

  protected string GetMessage()
        {
           
            IEnumerable<Product> products = new ShoppingCart
            {
                Products = new List<Product>
                {
                     new Product {Name ="Kayak",Category="Watersports",Price=275M},
                    new Product {Name ="Lifejacket",Category="Watersports",Price=48.95M},
                    new Product {Name ="Soccer ball",Category="Soccer",Price=19.5M},
                    new Product {Name ="Corner flag",Category="Soccer",Price=34.95M}
                }
            };
            decimal total = products.FilterByCategory("Soccer").TotalPrices();
            return String.Format("Soccer Total:{0:c}", total);
        }