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C# LINQ

本章将介绍 LINQ（Language Integrated Query），它是一组语言的特性，允许用 C# 语法直接查询各种数据源。

LINQ 概述​

什么是 LINQ？​

LINQ 是 C# 语言的一部分，提供了从多种数据源（集合、数据库、XML 等）查询数据的统一语法。

// LINQ 查询语法
int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// 查询表达式
var result = from n in numbers
             where n % 2 == 0
             select n * 2;

// 方法语法
var result2 = numbers
    .Where(n => n % 2 == 0)
    .Select(n => n * 2);

LINQ 组成​

组件说明
LINQ to Objects内存中的集合
LINQ to XMLXML 文档
LINQ to EntitiesEntity Framework
LINQ to SQLSQL Server

基本查询操作​

Where - 过滤​

int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// 方法语法
var evens = numbers.Where(n => n % 2 == 0);

// 查询语法
var evens2 = from n in numbers
             where n % 2 == 0
             select n;

Console.WriteLine(string.Join(", ", evens));  // 2, 4, 6, 8, 10

Select - 投影​

int[] numbers = { 1, 2, 3, 4, 5 };

// 转换
var squares = numbers.Select(n => n * n);

// 选择特定属性
class Person { public string Name { get; set; } public int Age { get; set; } }
Person[] people = { new Person { Name = "张三", Age = 20 } };

var names = people.Select(p => p.Name);

// 使用 SelectMany 展开嵌套集合
List<string>[] nested = new[] { new List<string> { "a", "b" }, new List<string> { "c", "d" } };
var flat = nested.SelectMany(list => list);

Select + Where 组合​

int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// 链式调用
var result = numbers
    .Where(n => n > 3)      // 先过滤
    .Select(n => n * 2)    // 再投影
    .Where(n => n < 20);    // 继续过滤

使用 Let 关键字（查询语法）​

int[] numbers = { 1, 2, 3, 4, 5, 6 };

var result = from n in numbers
             let square = n * n
             where square > 10
             select new { Original = n, Square = square };

排序操作​

OrderBy / OrderByDescending​

class Student
{
    public string Name { get; set; }
    public int Score { get; set; }
}

Student[] students = {
    new Student { Name = "张三", Score = 85 },
    new Student { Name = "李四", Score = 92 },
    new Student { Name = "王五", Score = 78 }
};

// 升序
var byScore = students.OrderBy(s => s.Score);

// 降序
var byScoreDesc = students.OrderByDescending(s => s.Score);

// 按多个字段排序
var multiOrder = students
    .OrderBy(s => s.Score)
    .ThenBy(s => s.Name);

Reverse​

int[] numbers = { 1, 2, 3, 4, 5 };
var reversed = numbers.Reverse();  // 5, 4, 3, 2, 1

分组操作​

GroupBy​

class Product
{
    public string Category { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
}

Product[] products = {
    new Product { Category = "食品", Name = "苹果", Price = 5 },
    new Product { Category = "食品", Name = "香蕉", Price = 3 },
    new Product { Category = "饮料", Name = "可乐", Price = 3 },
    new Product { Category = "饮料", Name = "果汁", Price = 5 }
};

// 按类别分组
var groups = products.GroupBy(p => p.Category);

foreach (var group in groups)
{
    Console.WriteLine($"类别: {group.Key}");
    foreach (var p in group)
    {
        Console.WriteLine($"  - {p.Name}: {p.Price}");
    }
}

按多个字段分组​

var groups = products.GroupBy(p => new { p.Category, p.Price > 5 ? "贵" : "便宜" });

GroupBy + 聚合​

var categoryStats = products
    .GroupBy(p => p.Category)
    .Select(g => new
    {
        Category = g.Key,
        Count = g.Count(),
        TotalValue = g.Sum(p => p.Price),
        AvgPrice = g.Average(p => p.Price)
    });

联接操作​

Join​

class Order
{
    public int Id { get; set; }
    public int CustomerId { get; set; }
    public decimal Total { get; set; }
}

class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Order[] orders = { new Order { Id = 1, CustomerId = 1, Total = 100 } };
Customer[] customers = { new Customer { Id = 1, Name = "张三" } };

// 联接查询
var result = from o in orders
             join c in customers on o.CustomerId equals c.Id
             select new { OrderId = o.Id, CustomerName = c.Name, Total = o.Total };

多表联接​

var result = from o in orders
             join c in customers on o.CustomerId equals c.Id
             join p in products on o.ProductId equals p.Id
             select new { Order = o, Customer = c, Product = p };

集合操作​

集合运算符​

int[] a = { 1, 2, 3, 4 };
int[] b = { 3, 4, 5, 6 };

// 并集
var union = a.Union(b);        // {1,2,3,4,5,6}

// 交集
var intersect = a.Intersect(b);  // {3,4}

// 差集
var except = a.Except(b);      // {1,2}

// 去重
var distinct = a.Distinct();   // {1,2,3,4,5,6}

集合判断​

int[] numbers = { 1, 2, 3, 4, 5 };

// 判断是否有元素
bool any = numbers.Any();           // true
bool anyEven = numbers.Any(n => n % 2 == 0);  // true

// 判断是否所有元素都满足
bool allPositive = numbers.All(n => n > 0);   // true

// 判断是否包含
bool contains = numbers.Contains(3);   // true

元素操作​

First / Last​

int[] numbers = { 1, 2, 3, 4, 5 };

var first = numbers.First();              // 1
var firstEven = numbers.First(n => n % 2 == 0);  // 2

var last = numbers.Last();                // 5

// 找不到时返回默认值
var firstOrDefault = numbers.FirstOrDefault();   // 1
var firstOrDefault2 = numbers.FirstOrDefault(n => n > 10);  // 0

ElementAt / Single​

int[] numbers = { 1, 2, 3, 4, 5 };

var element = numbers.ElementAt(2);  // 3

// Single：返回唯一元素（没有或多个会抛异常）
int[] single = { 5 };
var singleElement = single.Single();  // 5

聚合操作​

Count / Sum / Average​

int[] numbers = { 1, 2, 3, 4, 5 };

int count = numbers.Count();           // 5
int sum = numbers.Sum();               // 15
double avg = numbers.Average();        // 3
int max = numbers.Max();               // 5
int min = numbers.Min();               // 1

Aggregate - 聚合​

int[] numbers = { 1, 2, 3, 4, 5 };

// 累加器
int product = numbers.Aggregate((acc, n) => acc * n);  // 120 (1*2*3*4*5)

// 带初始值
int sum = numbers.Aggregate(100, (acc, n) => acc + n);  // 115 (100+1+2+3+4+5)

分区操作​

int[] numbers = { 1, 2, 3, 4, 5, 6 };

// Take：取前几个
var take3 = numbers.Take(3);    // {1,2,3}

// Skip：跳过前几个
var skip2 = numbers.Skip(2);   // {3,4,5,6}

// TakeWhile：满足条件时取
var takeWhile = numbers.TakeWhile(n => n < 4);  // {1,2,3}

// SkipWhile：跳过满足条件的
var skipWhile = numbers.SkipWhile(n => n < 4); // {4,5,6}

转换操作​

ToArray / ToList / ToDictionary​

var list = new List<int> { 1, 2, 3 };

int[] array = list.ToArray();   // 转换为数组
List<int> newList = array.ToList();  // 转换为列表

// 转换为字典
class Product { public string Code { get; set; } public string Name { get; set; } }
var products = new Product[] { new Product { Code = "A01", Name = "苹果" } };

Dictionary<string, string> dict = products.ToDictionary(p => p.Code, p => p.Name);

ToLookup​

// 类似 GroupBy，返回 ILookup
var lookup = products.ToLookup(p => p.Category);

foreach (var group in lookup)
{
    Console.WriteLine(group.Key);
    foreach (var p in group)
        Console.WriteLine($"  {p.Name}");
}

延迟执行​

延迟 vs 立即执行​

int[] numbers = { 1, 2, 3, 4, 5 };

// 延迟执行：查询不会立即执行
var query = numbers.Where(n => n > 2).Select(n => n * 2);

// 修改原集合
numbers[0] = 100;

// 真正执行时才会求值
foreach (var n in query)
    Console.WriteLine(n);  // 输出: 6, 8, 10（100被过滤掉了）

立即执行​

// ToList() 等操作会立即执行
var list = numbers.Where(n => n > 2).ToList();

// Count 也会立即执行
int count = numbers.Count(n => n > 2);

小结​

  1. 查询方法：Where、Select、OrderBy、GroupBy
  2. 联接操作：Join、GroupJoin
  3. 集合操作：Union、Intersect、Except
  4. 聚合操作：Sum、Average、Max、Min、Count
  5. 延迟执行：LINQ 查询是延迟执行的
  6. 转换操作：ToArray、ToList、ToDictionary

练习​

  1. 使用 LINQ 实现学生成绩统计（平均分、最高分）
  2. 实现商品分类统计（每类商品数量、总价值）
  3. 使用 LINQ 实现排行榜功能（排序、分页）
  4. 实现多表联接查询（订单、客户、商品）