C# Lambda 表达式与 LINQ

18.1 Lambda 表达式

基本语法

// 参数列表 => 表达式
(参数) => 表达式

// 无参数
() => Console.WriteLine("Hello")

// 一个参数
x => x * x

// 多个参数
(x, y) => x + y

// 多行语句
(x, y) =>
{
    int result = x + y;
    return result;
}

示例

// 使用 Func
Func<int, int> square = x => x * x;
Console.WriteLine(square(5));  // 25

// 使用 Action
Action<string> greet = name => Console.WriteLine($"Hello, {name}");
greet("Alice");

// 多行 lambda
Func<int, int, int> calculate = (a, b) =>
{
    int sum = a + b;
    int product = a * b;
    return sum + product;
};

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18.2 LINQ 概述

LINQ（Language Integrated Query）是 C# 中强大的数据查询功能。

数据源

// 数组
int[] numbers = { 5, 2, 8, 1, 9 };

// 列表
List<string> fruits = new() { "apple", "banana", "cherry" };

// 数据库（Entity Framework）
// var users = db.Users.Where(u => u.Age > 18);

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18.3 查询语法

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

// 基本查询
var evenNumbers = from n in numbers
                  where n % 2 == 0
                  orderby n
                  select n;

// 输出: 2, 4, 6, 8
foreach (var n in evenNumbers)
{
    Console.Write($"{n} ");
}

// 投影
var squares = from n in numbers
              where n > 3
              select new { Number = n, Square = n * n };

foreach (var item in squares)
{
    Console.WriteLine($"{item.Number}² = {item.Square}");
}

// 分组
string[] words = { "apple", "banana", "cherry", "avocado", "blueberry" };

var groups = from w in words
             group w by w[0] into g
             orderby g.Key
             select new { FirstLetter = g.Key, Words = g };

foreach (var group in groups)
{
    Console.WriteLine($"{group.FirstLetter}: {string.Join(", ", group.Words)}");
}

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18.4 方法语法（推荐）

基本操作

List<int> numbers = new() { 5, 2, 8, 1, 9, 3, 7, 4, 6 };

// Where 过滤
var evens = numbers.Where(n => n % 2 == 0);  // 2, 4, 6, 8

// Select 投影
var squared = numbers.Select(n => n * n);    // 25, 4, 64, ...

// OrderBy 排序
var sorted = numbers.OrderBy(n => n);         // 1, 2, 3, 4, 5, 6, 7, 8, 9
var desc = numbers.OrderByDescending(n => n); // 9, 8, 7, 6, 5, 4, 3, 2, 1

// ThenBy 次级排序
var words = new[] { "apple", "Cat", "banana", "Dog" };
var sortedWords = words.OrderBy(w => w.Length).ThenBy(w => w);

聚合操作

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
int first = numbers.First();              // 1
int firstOrDefault = numbers.FirstOrDefault(n => n > 10);  // 0
int single = numbers.Single(n => n == 3); // 3

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18.5 常用操作符

Filter - Where

var adults = people.Where(p => p.Age >= 18);
var games = products.Where(p => p.Category == "Game" && p.Price < 100);

Projection - Select

// 选择单个属性
var names = users.Select(u => u.Name);

// 匿名类型
var infos = users.Select(u => new { u.Name, u.Age });

// 索引
var indexed = words.Select((w, i) => $"{i}: {w}");

Element - First/Last/Single

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

int first = numbers.First();                    // 1
int last = numbers.Last();                      // 5
int single = numbers.Single(n => n == 3);       // 3
int firstEven = numbers.First(n => n % 2 == 0); // 2

// 安全的版本
int? maybe = numbers.FirstOrDefault(n => n > 10);  // 0

Quantifier - All/Any/Contains

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

bool allPositive = numbers.All(n => n > 0);       // True
bool anyEven = numbers.Any(n => n % 2 == 0);       // True
bool hasThree = numbers.Contains(3);               // True

Set - Distinct/Union/Intersect

int[] numbers1 = { 1, 2, 3, 4, 5 };
int[] numbers2 = { 4, 5, 6, 7, 8 };

var distinct = numbers1.Distinct();           // 1, 2, 3, 4, 5
var union = numbers1.Union(numbers2);         // 1, 2, 3, 4, 5, 6, 7, 8
var intersect = numbers1.Intersect(numbers2); // 4, 5
var except = numbers1.Except(numbers2);       // 1, 2, 3

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18.6 LINQ to Objects

实际操作

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

List<Student> students = new()
{
    new() { Name = "Alice", Score = 85, Class = "A" },
    new() { Name = "Bob", Score = 92, Class = "B" },
    new() { Name = "Charlie", Score = 78, Class = "A" },
    new() { Name = "David", Score = 95, Class = "B" },
};

// 各班平均分
var classAverages = students
    .GroupBy(s => s.Class)
    .Select(g => new
    {
        Class = g.Key,
        Average = g.Average(s => s.Score),
        Count = g.Count()
    })
    .OrderByDescending(x => x.Average);

foreach (var item in classAverages)
{
    Console.WriteLine($"{item.Class}班: 平均{item.Average:F1}分 ({item.Count}人)");
}

// 前 3 名
var top3 = students
    .OrderByDescending(s => s.Score)
    .Take(3)
    .Select((s, i) => $"第{i + 1}名: {s.Name} - {s.Score}分");

foreach (var item in top3)
{
    Console.WriteLine(item);
}

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18.7 延迟执行

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

// LINQ 查询不会立即执行
var query = numbers.Where(n =>
{
    Console.WriteLine($"检查: {n}");
    return n > 2;
});

Console.WriteLine("查询已定义，未执行");

// 遍历时执行
foreach (var n in query)
{
    Console.WriteLine($"结果: {n}");
}

// 强制立即执行
var list = query.ToList();   // 立即执行
var array = query.ToArray(); // 立即执行
int count = query.Count();   // 立即执行

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18.8 本章小结

• Lambda 表达式：参数 => 表达式
• LINQ 有查询语法和方法语法两种形式
• 方法语法更常用：Where、Select、OrderBy 等
• 聚合操作：Count、Sum、Average、Max、Min
• 分组使用 GroupBy
• LINQ 查询延迟执行，遍历或强制转换时执行

练习题

1. 从集合中筛选出所有偶数并排序
2. 对人员列表按年龄分组
3. 计算成绩列表的平均分和最高分
4. 以下代码输出什么？

   int[] nums = { 1, 2, 3, 4, 5 };
   var q = nums.Where(n => n > 3).Select(n => n * n);
   Console.WriteLine(string.Join(",", q));