C# LINQ常用语法及实际应用示例

from 变量 in 数据源
where 条件
orderby 字段
select 结果

数据源.方法(参数 => 表达式)

// 查询表达式
var result = from num in numbers
             where num > 5
             select num;
// 方法语法
var result = numbers.Where(num => num > 5);

// 升序
var result = numbers.OrderBy(num => num);
// 降序
var result = numbers.OrderByDescending(num => num);
// 多字段排序
var result = students.OrderBy(s => s.Age)
                    .ThenBy(s => s.Name);

// 选择特定字段
var names = students.Select(s => s.Name);
// 创建新对象
var studentInfo = students.Select(s => new {
    Name = s.Name,
    Age = s.Age
});

// 取前N个
var top3 = numbers.Take(3);
// 跳过前N个
var afterFirst3 = numbers.Skip(3);
// 判断是否存在
bool hasEven = numbers.Any(n => n % 2 == 0);
// 判断是否所有元素都满足条件
bool allPositive = numbers.All(n => n > 0);
// 获取第一个元素
var first = numbers.First();
// 获取最后一个元素
var last = numbers.Last();
// 计数
int count = numbers.Count();
// 求和
int sum = numbers.Sum();
// 最大值
int max = numbers.Max();
// 最小值
int min = numbers.Min();
// 平均值
double average = numbers.Average();

class Student
{
    public string Name { get; set; }
    public int Age { get; set; }
    public int Score { get; set; }
}
List<Student> students = new List<Student>
{
    new Student { Name = "Alice", Age = 20, Score = 85 },
    new Student { Name = "Bob", Age = 22, Score = 90 },
    new Student { Name = "Charlie", Age = 19, Score = 78 }
};

// 1. 筛选年龄大于20的学生
var olderStudents = students.Where(s => s.Age > 20);
// 2. 按分数降序排序
var sortedByScore = students.OrderByDescending(s => s.Score);
// 3. 选择学生姓名和分数
var nameScores = students.Select(s => new { 
    s.Name, 
    s.Score 
});
// 4. 获取分数最高的学生
var topStudent = students.OrderByDescending(s => s.Score).First();
// 5. 计算平均分
var averageScore = students.Average(s => s.Score);
// 6. 分组查询
var groupByAge = students.GroupBy(s => s.Age);

csharp
int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
// 获取偶数
var evenNumbers = numbers.Where(n => n % 2 == 0);
// 平方每个数
var squares = numbers.Select(n => n * n);
// 获取大于5的数并排序
var result = numbers.Where(n => n > 5)
                   .OrderBy(n => n);

string[] names = { "Alice", "Bob", "Charlie", "David" };
// 查找以A开头的名字
var aNames = names.Where(name => name.StartsWith("A"));
// 按长度排序
var sortedByLength = names.OrderBy(name => name.Length);