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

 1 using System.Collections;
 2 using System.ComponentModel;
 3
 4 namespace Chapter01.CSharp1
 5 {
 6     [Description("Listing 1.01")]
 7     public class Product
 8     {
 9         string name;
10         public string Name
11         {
12             get { return name; }
13         }
14
15         decimal price;
16         public decimal Price
17         {
18             get { return price; }
19         }
20
21         public Product(string name, decimal price)
22         {
23             this.name = name;
24             this.price = price;
25         }
26
27         public static ArrayList GetSampleProducts()
28         {
29             ArrayList list = new ArrayList();
30             list.Add(new Product("West Side Story", 9.99m));
31             list.Add(new Product("Assassins", 14.99m));
32             list.Add(new Product("Frogs", 13.99m));
33             list.Add(new Product("Sweeney Todd", 10.99m));
34             return list;
35         }
36
37         public override string ToString()
38         {
39             return string.Format("{0}: {1}", name, price);
40         }
41     }
42 }

ArrayListQuery类

 1 using System;
 2 using System.Collections;
 3 using System.ComponentModel;
 4
 5 namespace Chapter01.CSharp1
 6 {
 7     [Description("Listing 1.10")]
 8     class ArrayListQuery
 9     {
10         static void Main()
11         {
12             ArrayList products = Product.GetSampleProducts();
13             foreach (Product product in products)
14             {
15                 if (product.Price > 10m)
16                 {
17                     Console.WriteLine(product);
18                 }
19             }
20         }
21     }
22 }

2.测试和打印分开

C#2.0

product类

 1 using System.Collections.Generic;
 2 using System.ComponentModel;
 3
 4 namespace Chapter01.CSharp2
 5 {
 6     [Description("Listing 1.02")]
 7     public class Product
 8     {
 9         string name;
10         public string Name
11         {
12             get { return name; }
13             private set { name = value; }
14         }
15
16         decimal price;
17         public decimal Price
18         {
19             get { return price; }
20             private set { price = value; }
21         }
22
23         public Product(string name, decimal price)
24         {
25             Name = name;
26             Price = price;
27         }
28
29         public static List<Product> GetSampleProducts()
30         {
31             List<Product> list = new List<Product>();
32             list.Add(new Product("West Side Story", 9.99m));
33             list.Add(new Product("Assassins", 14.99m));
34             list.Add(new Product("Frogs", 13.99m));
35             list.Add(new Product("Sweeney Todd", 10.99m));
36             return list;
37         }
38
39         public override string ToString()
40         {
41             return string.Format("{0}: {1}", name, price);
42         }
43     }
44 }

ListQueryWithDelegates类

 1 using System;
 2 using System.Collections.Generic;
 3 using System.ComponentModel;
 4
 5 namespace Chapter01.CSharp2
 6 {
 7     [Description("Listing 1.11")]
 8     class ListQueryWithDelegates
 9     {
10         static void Main()
11         {
12             List<Product> products = Product.GetSampleProducts();
13             Predicate<Product> test = delegate(Product p)
14                 { return p.Price > 10m; };
15             List<Product> matches = products.FindAll(test);
16
17             Action<Product> print = Console.WriteLine;
18             matches.ForEach(print);
19         }
20     }
21 }

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

 1 using System;
 2 using System.Collections.Generic;
 3 using System.ComponentModel;
 4
 5 namespace Chapter01.CSharp2
 6 {
 7     [Description("Listing 1.12")]
 8     class ListQueryWithDelegatesCompact
 9     {
10         static void Main()
11         {
12             List<Product> products = Product.GetSampleProducts();
13             products.FindAll(delegate(Product p) { return p.Price > 10; })
14                     .ForEach(delegate(Product p) { Console.WriteLine(p); });
15         }
16     }
17 }

3.用lambda表达式来测试

C#3.0

product

 1 using System.Collections.Generic;
 2 using System.ComponentModel;
 3
 4 namespace Chapter01.CSharp3
 5 {
 6     [Description("Listing 1.3")]
 7     class Product
 8     {
 9         public string Name { get; private set; }
10         public decimal Price { get; private set; }
11
12         public Product(string name, decimal price)
13         {
14             Name = name;
15             Price = price;
16         }
17
18         Product()
19         {
20         }
21
22         public static List<Product> GetSampleProducts()
23         {
24             return new List<Product>
25             {
26                 new Product { Name="West Side Story", Price = 9.99m },
27                 new Product { Name="Assassins", Price=14.99m },
28                 new Product { Name="Frogs", Price=13.99m },
29                 new Product { Name="Sweeney Todd", Price=10.99m}
30             };
31         }
32
33         public override string ToString()
34         {
35             return string.Format("{0}: {1}", Name, Price);
36         }
37     }
38 }

ListQueryWithLambdaExpression类

 1 using System;
 2 using System.Collections.Generic;
 3 using System.ComponentModel;
 4 using System.Linq;
 5
 6 namespace Chapter01.CSharp3
 7 {
 8     [Description("Listing 1.13")]
 9     class ListQueryWithLambdaExpression
10     {
11         static void Main()
12         {
13             List<Product> products = Product.GetSampleProducts();
14             foreach (Product product in products.Where(p => p.Price > 10))
15             {
16                 Console.WriteLine(product);
17             }
18         }
19     }
20 }

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）。

原文地址：https://www.cnblogs.com/csxukang/p/9290854.html