LINQ概讲

>d 实现了IEnumerable\<T>接口的类都可以使用LINQ中的方法

```mindmap>500
# LINQ
## 数据过滤 where()
## 数据条数 count()
## 至少一条满足bool？Any()
## 获取一条数据
### single
### SingleOrDefault
### First
### FirstOrDefault
## 排序 OrderBy
## 限制结果集 Skip
## 聚合函数 Max、Min、Avg、Sum、Count
## 分组 GroupBy
## 投影 select
## 集合转换 ToArray、ToList

```
# where()
```java
IEnumerable<Employee> list1 = list.Where(e => e.Salary > 2500 && e.Age < 35);
```
# cout()
```java
int count1 = list.Count(e => e.Salary > 5000 || e.Age < 30);
int count2 = list.Where(e => e.Salary > 5000 || e.Age < 30).Count();
```
# Any()
```java
//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();
```
# Single、SingleOrDefault、First、FirstOrDefault
| 方法| 解释 |
|-----------------|-------------------------------------|
| Single          | 有且只有一条数据，如果没有数据或者数据大于1，则报错|
| SingleOrDefault | 最多只有一条数据，如果没有数据，则返回类型默认值，如果多余1条，则报错 |
| First           | 有一条或多条，则返回第一个，无数据则报错|
| FirstOrDefault  | 有数据，则返回第一个，否则返回了类型默认值|
```java
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);

```
# OrderBy
```java
var orderedItems1 = list.OrderBy(e => e.Age);//正序
var orderedItems2 = list.OrderByDescending(e => e.Salary);//倒叙
```
# 限制结果集 Skip
```java
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
```
# Max、Min、Avg、Sum、Count
```java
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);
```
# GroupBy
返回值为IEnumerable<IGrouping<TKey, TSource>>类型TKey和分组条件表达式类型一致

```java
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

```java
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}");
}

```