排序运算

一、排序运算方法

在查询语法中，通过 orderby 对序列进行排序，第一个排序条件对元素执行主要排序，后续排序为次要排序，比如：

IEnumerable<Country> querySortedCountries =
    from country in countries
    // Area 主要排序，Population 次要排序
    orderby country.Area, country.Population descending 
    select country;

在方法语法中，主要排序和次要排序具有各自的方法可供调用，下表列出了对数据进行排序的标准查询运算符方法：

方法	说明	查询语法	连接
OrderBy	按升序对值排序	orderby	Enumerable.OrderBy 、Queryable.OrderBy
OrderByDescending	按降序对值排序	orderby ... descending	Enumerable.OrderByDescending 、Queryable.OrderByDescending
ThenBy	按升序执行次要排序	orderby …, …	Enumerable.ThenBy 、Queryable.ThenBy
ThenByDescending	按降序执行次要排序	orderby …, … descending	Enumerable.ThenByDescending 、Queryable.ThenByDescending
Reverse	反转集合中元素的顺序	×	Enumerable.Reverse 、Queryable.Reverse

二、OrderBy

OrderBy() 方法可以实现按值的升序排序，方法返回排序后的可迭代结果集。下面的代码展示了查询语法和方法语法按值升序排序的使用：

string[] words = { "the", "quick", "brown", "fox", "jumps" };

// 查询语法
IEnumerable<string> query1 = from word in words
                            orderby word.Length
                            select word;

foreach (string str in query1)
    Console.Write(str + " ");

Console.WriteLine();

// 方法语法
IEnumerable<string> query2 = words.OrderBy(word => word.Length);

foreach (string str in query2)
    Console.Write(str + " ");

/* This code produces the following output:  
    the fox quick brown jumps
    the fox quick brown jumps  
*/

三、OrderByDescending

OrderByDescending() 方法可以实现按值的降序排序，方法返回排序后的可迭代结果集。下面的代码展示了查询语法和方法语法按值降序排序的使用：

string[] words = { "the", "quick", "brown", "fox", "jumps" };

IEnumerable<string> query1 = from word in words
                             orderby word.Length descending
                             select word;

foreach (string str in query1)
    Console.Write(str + " ");

Console.WriteLine();

IEnumerable<string> query2 = words.OrderByDescending(word => word.Length);

foreach (string str in query2)
    Console.Write(str + " ");

/* This code produces the following output:  
    quick brown jumps the fox
    quick brown jumps the fox
*/

四、ThenBy

ThenBy() 方法实现次要排序升序，在查询语法中，会对非首个属性进行次要排序，在方法语法中通过该方法完成，次要排序默认升序。下面的代码展示了查询语法和方法语法对次要排序的使用：

string[] words = { "the", "quick", "brown", "fox", "jumps" };

IEnumerable<string> query = from word in words
                            orderby word.Length, word.Substring(0, 1)
                            select word;

foreach (string str in query)
    Console.Write(str + " ");

Console.WriteLine();

IEnumerable<string> query2 = words
    .OrderBy(word => word.Length)
    .ThenBy(word => word.Substring(0, 1));

foreach (string str in query2)
    Console.Write(str + " ");

/* This code produces the following output:  
    fox the brown jumps quick
    fox the brown jumps quick
*/

五、ThenByDescending

ThenByDescending() 方法实现次要排序降序，在查询语法中，会对非首个属性进行次要排序，通过descending 关键字实现降序，在方法语法中通过该方法完成。下面的代码展示了查询语法和方法语法对次要排序降序的使用：

string[] words = { "the", "quick", "brown", "fox", "jumps" };

IEnumerable<string> query = from word in words
                        orderby word.Length, word.Substring(0, 1) descending
                        select word;

foreach (string str in query)
    Console.Write(str + " ");

Console.WriteLine();

IEnumerable<string> query2 = words
    .OrderBy(word => word.Length)
    .ThenByDescending(word => word.Substring(0, 1));

foreach (string str in query2)
    Console.Write(str + " ");

/* This code produces the following output:  
    the fox quick jumps brown
    the fox quick jumps brown
*/

六、Reverse

Reverse() 方法可以实现对序列的反转。

string[] words = { "the", "quick", "brown", "fox", "jumps" };

foreach (string str in words)
    Console.Write(str + " ");

Console.WriteLine();

// 反转列表
IEnumerable<string> query = words.Reverse();

foreach (string str in query)
    Console.Write(str + " ");

/*
 This code produces the following output:
    the quick brown fox jumps
    jumps fox brown quick the
*/