C#教程之C#语言各个版本特性

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C#教程之C#语言各个版本特性

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 > 10; })
                   .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 > 10))
           {
               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）。

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