20121009 P85

Essential Language Features

扩展方法：

1、下面是通过静态类的静态属性以及this来实现单独一个类的方法来的扩展

using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace ConsoleApplication1{    using System.Collections.Generic;    public class Product     {        public int ProductID { get; set; }        private string name;        public string Description { get; set;}        public decimal Price { get; set; }        public string Category { set; get;}        public string Name        {            get { return ProductID + name;}            set { name = value; }        }    }    public class ShoppingCart    {        public List<Product> Products { get; set; }    }    /// <summary>    /// 下面的参数是带this的，类是静态类，怎么扩展一个类的方法    /// </summary>    public static class MyExtensionMethods    {        public static decimal TotalPrices(this ShoppingCart cartParam)        {            decimal total = 0;            foreach (Product prod in cartParam.Products)            {                total += prod.Price;            }            return total;        }    }    class Program    {        static void Main(string[] args)        {            // create and populate ShoppingCart            ShoppingCart cart = new ShoppingCart            {                Products = new List<Product>                 {                    new Product {Name = "Kayak", Price = 275M},                    new Product {Name = "Lifejacket", Price = 48.95M},                    new Product {Name = "Soccer ball", Price = 19.50M},                    new Product {Name = "Corner flag", Price = 34.95M}                }            };            // get the total value of the products in the cart            decimal cartTotal = cart.TotalPrices();            Console.WriteLine("Total: {0:c}", cartTotal);            Console.ReadKey();        }    }}

利用this和静态类来实现方法的扩展。this标志了TotalPrices方法所在的类。不好的地方是只能用在一个类中，也就是只能指定一个类。所以我们通过接口避免此弊端。

2、通过接口实现扩展方法

using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace ConsoleApplication1{    using System.Collections.Generic;    using System.Collections;    public class Product     {        public int ProductID { get; set; }        private string name;        public string Description { get; set;}        public decimal Price { get; set; }        public string Category { set; get;}        public string Name        {            get { return ProductID + name;}            set { name = value; }        }    }    public class ShoppingCart : IEnumerable<Product>    {        public List<Product> Products { get; set; }        public IEnumerator<Product> GetEnumerator()        {            return Products.GetEnumerator();//对Products进行迭代获取Product列表        }        IEnumerator IEnumerable.GetEnumerator()        {            return GetEnumerator();        }    }    /// <summary>    /// 下面的参数是带this的，类是静态类，怎么扩展一个类的方法    /// </summary>    public static class MyExtensionMethods    {        public static decimal TotalPrices(this IEnumerable<Product> productEnum)        {            decimal total = 0;            foreach (Product prod in productEnum)            {                total += prod.Price;            }            return total;        }    }    class Program    {        static void Main(string[] args)        {            IEnumerable<Product> products = new ShoppingCart            {                Products = new List<Product>                 {                    new Product {Name = "Kayak", Price = 275M},                    new Product {Name = "Lifejacket", Price = 48.95M},                    new Product {Name = "Soccer ball", Price = 19.50M},                    new Product {Name = "Corner flag", Price = 34.95M}                }            };             //数组也实现了枚举类型的接口。            Product[] productArray =             {                new Product {Name = "Kayak", Price = 275M},                new Product {Name = "Lifejacket", Price = 48.95M},                new Product {Name = "Soccer ball", Price = 19.50M},                new Product {Name = "Corner flag", Price = 34.95M}            };             decimal cartTotal = products.TotalPrices();            decimal arrayTotal = productArray.TotalPrices();            Console.WriteLine("Cart Total: {0:c}", cartTotal);            Console.WriteLine("Array Total: {0:c}", arrayTotal);            Console.ReadKey();        }    }}

数组实现IEnumerable<Product>接口是要注意的地方。除此之外，该方法还能带额外的参数。

3、创造过滤方法

在静态类中添加代码

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

在main函数中，添加代码：

 IEnumerable<Product> products1 = 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 = 19.50M},                new Product {Name = "Corner flag", Category = "Soccer", Price = 34.95M}                }            };            foreach (Product prod in products1.FilterByCategory("Soccer"))            {                Console.WriteLine("Name: {0}, Price {1:c}", prod.Name, prod.Price);            }

4.lamda表达式