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）。