2.3 LINQ

2.3 LINQ

实现了IEnumerable<T>接口的类都可以使用LINQ中的方法。

数据过滤 where()

IEnumerable<Employee> list1 = list.Where(e => e.Salary > 2500 && e.Age < 35);
foreach (Employee e in list1)
{
    Console.WriteLine(e);
}

获取数据条数 count()

int count1 = list.Count(e => e.Salary > 5000 || e.Age < 30);
int count2 = list.Where(e => e.Salary > 5000 || e.Age < 30).Count();

至少一条满足条件的数据Any()

//只要有符合的数据就停止
bool b1 = list.Any(e => e.Salary > 8000);
bool b2 = list.Where(e => e.Salary > 8000).Any();

获取一条数据Single、SingleOrDefault、First、FirstOrDefault

Employee e1 = list.Single(e => e.Id == 6);
Console.WriteLine(e1);
Employee? e2 = list.SingleOrDefault(e => e.Id == 9);
if (e2 == null)
{
    Console.WriteLine("没有Id==9的数据");
}
else
{
    Console.WriteLine(e2);
}
Employee e3 = list.First(e => e.Age > 30);
Console.WriteLine(e3);
Employee? e4 = list.FirstOrDefault(e => e.Age > 30);
if (e4 == null)
{
    Console.WriteLine("没有大于30岁的数据");
}
else
{
    Console.WriteLine(e2);
}
Employee e5 = list.First(e => e.Salary > 9999);

排序 OrderBy

var orderedItems1 = list.OrderBy(e => e.Age);//正序
var orderedItems2 = list.OrderByDescending(e => e.Salary);//倒叙

限制结果集 Skip

var orderedItems1 = list.Skip(2).Take(3);
list.Skip(2);//跳过2条数据
list.Take(2);//获取list前2条数据

聚合函数 Max、Min、Avg、Sum、Count

int maxAge = list.Max(e => e.Age);
long minId = list.Min(e => e.Id);
int minSalary2 = list.Where(e => e.Age > 30).Min(e => e.Salary);

分组 GroupBy

返回值为IEnumerable<IGrouping<TKey, TSource>>类型TKey和分组条件表达式类型一致

IEnumerable<IGrouping<int, Employee>> items = list.GroupBy(e => e.Age);
foreach (IGrouping<int, Employee> item in items)
{
    int age = item.Key;
    int count = item.Count();
    int maxSalary = item.Max(e => e.Salary);
    double avgSalary = item.Average(e => e.Salary);
    Console.WriteLine($"年龄{item.Key},人数{count},最高工资{maxSalary},平均工资{avgSalary}");
}

投影 select

IEnumerable<string> names = list.Select(e => e.Gender ? "男" : "女");//将bool转换为男女
//使用匿名属性
var items = list.Select(e => new { e.Name, e.Age, XingBie = e.Gender ? "男" : "女" });
foreach (var item in items)
{
    string name = item.Name;
    int age = item.Age;
    string xingbie = item.XingBie;
    Console.WriteLine($"名字={name},年龄={age},性别={xingbie}");
}

集合转换 ToArray、ToList

查询语法

var items2 = from e in list
            where e.Salary > 3000
            orderby e.Age
            select new { e.Name, e.Age, Gender = e.Gender ? "男" : "女" };