C#语言各个版本特性(三)

三、查询集合

1.找出List<Product>列表中符合特定条件的所有元素

C#1.1 查询步骤：循环，if判断，打印

product类

 using System.Collections;

 using System.ComponentModel;

 namespace Chapter01.CSharp1

 {

     [Description("Listing 1.01")]

     public class Product

     {

         string name;

         public string Name

         {

             get { return name; }

         }

         decimal price;

         public decimal Price

         {

             get { return price; }

         }

         public Product(string name, decimal price)

         {

             this.name = name;

             this.price = price;

         }

         public static ArrayList GetSampleProducts()

         {

             ArrayList list = new ArrayList();

             list.Add(new Product("West Side Story", 9.99m));

             list.Add(new Product("Assassins", 14.99m));

             list.Add(new Product("Frogs", 13.99m));

             list.Add(new Product("Sweeney Todd", 10.99m));

             return list;

         }

         public override string ToString()

         {

             return string.Format("{0}: {1}", name, price);

         }

     }

 }

ArrayListQuery类

 using System;

 using System.Collections;

 using System.ComponentModel;

 namespace Chapter01.CSharp1

 {

     [Description("Listing 1.10")]

     class ArrayListQuery

     {

         static void Main()

         {

             ArrayList products = Product.GetSampleProducts();

             foreach (Product product in products)

             {

                 if (product.Price > 10m)

                 {

                     Console.WriteLine(product);

                 }

             }

         }

     }

 }

2.测试和打印分开

C#2.0

product类

 using System.Collections.Generic;

 using System.ComponentModel;

 namespace Chapter01.CSharp2

 {

     [Description("Listing 1.02")]

     public class Product

     {

         string name;

         public string Name

         {

             get { return name; }

             private set { name = value; }

         }

         decimal price;

         public decimal Price

         {

             get { return price; }

             private set { price = value; }

         }

         public Product(string name, decimal price)

         {

             Name = name;

             Price = price;

         }

         public static List<Product> GetSampleProducts()

         {

             List<Product> list = new List<Product>();

             list.Add(new Product("West Side Story", 9.99m));

             list.Add(new Product("Assassins", 14.99m));

             list.Add(new Product("Frogs", 13.99m));

             list.Add(new Product("Sweeney Todd", 10.99m));

             return list;

         }

         public override string ToString()

         {

             return string.Format("{0}: {1}", name, price);

         }

     }

 }

ListQueryWithDelegates类

 using System;

 using System.Collections.Generic;

 using System.ComponentModel;

 namespace Chapter01.CSharp2

 {

     [Description("Listing 1.11")]

     class ListQueryWithDelegates

     {

         static void Main()

         {

             List<Product> products = Product.GetSampleProducts();

             Predicate<Product> test = delegate(Product p)

                 { return p.Price > 10m; };

             List<Product> matches = products.FindAll(test);

             Action<Product> print = Console.WriteLine;

             matches.ForEach(print);

         }

     }

 }

变量test的初始化使用了匿名方法，而print变量的初始化使用了方法组转换，它简化了从现有方法创建委托的过程。不仅简单而且强大！

ListQueryWithDelegatesCompact类

 using System;

 using System.Collections.Generic;

 using System.ComponentModel;

 namespace Chapter01.CSharp2

 {

     [Description("Listing 1.12")]

     class ListQueryWithDelegatesCompact

     {

         static void Main()

         {

             List<Product> products = Product.GetSampleProducts();

             products.FindAll(delegate(Product p) { return p.Price > ; })

                     .ForEach(delegate(Product p) { Console.WriteLine(p); });

         }

     }

 }

3.用lambda表达式来测试

C#3.0

product

 using System.Collections.Generic;

 using System.ComponentModel;

 namespace Chapter01.CSharp3

 {

     [Description("Listing 1.3")]

     class Product

     {

         public string Name { get; private set; }

         public decimal Price { get; private set; }

         public Product(string name, decimal price)

         {

             Name = name;

             Price = price;

         }

         Product()

         {

         }

         public static List<Product> GetSampleProducts()

         {

             return new List<Product>

             {

                 new Product { Name="West Side Story", Price = 9.99m },

                 new Product { Name="Assassins", Price=14.99m },

                 new Product { Name="Frogs", Price=13.99m },

                 new Product { Name="Sweeney Todd", Price=10.99m}

             };

         }

         public override string ToString()

         {

             return string.Format("{0}: {1}", Name, Price);

         }

     }

 }

ListQueryWithLambdaExpression类

 using System;

 using System.Collections.Generic;

 using System.ComponentModel;

 using System.Linq;

 namespace Chapter01.CSharp3

 {

     [Description("Listing 1.13")]

     class ListQueryWithLambdaExpression

     {

         static void Main()

         {

             List<Product> products = Product.GetSampleProducts();

             foreach (Product product in products.Where(p => p.Price > ))

             {

                 Console.WriteLine(product);

             }

         }

     }

 }

总结：

→C#1,条件和操作紧密耦合两者都是硬编码的

→C#2,条件和操作分开，匿名方法使委托变得简单（匿名方法有助于问题的可分离性）

→C#3Lambda表达式使条件变得更容易阅读（Lambda表达式增强了可读性）。

巴特西

C#语言各个版本特性(三)

最新文章

热门文章