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);