常用方法

```java
public record Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Gender { get; set; }
    public int Score { get; set; }
}
```

```java
using MY_06_LINQ;

List<Student> list = new List<Student>
{
    new Student{Id=202201,Name="Mike",Gender=true,Score=90},
    new Student{Id=202202,Name="Linda",Gender=false,Score=85},
    new Student{Id=202203,Name="Lucy",Gender=false,Score=88},
    new Student{Id=202204,Name="Mary",Gender=true,Score=90},
    new Student{Id=202205,Name="Lisa",Gender=false,Score=95},
    new Student{Id=202206,Name="Amy",Gender=true,Score=80},
    new Student{Id=202207,Name="Peter",Gender=true,Score=90},
    new Student{Id=202208,Name="Polo",Gender=true,Score=95}
};
```
# 1.数据过滤
```java
IEnumerable<Student> students = list.Where(x => x.Score > 85);
foreach (Student student in students)
{
    Console.WriteLine(student);
}
Console.WriteLine("--------------------------");
IEnumerable<Student> students2 = list.Where(x => x.Score > 85 && x.Gender);
foreach (Student student in students2)
{
    Console.WriteLine(student);
}
```
# 2.获取数据记录
```java
int count1=list.Count();
int count2 = list.Count(x => x.Score > 85);
int count3=list.Where(x=>x.Score>85&&x.Gender).Count();
Console.WriteLine($"count1={count1},    count2={count2},    count3={count3}");
```
# 3.获取第一条数据
```java
Student student1 = list.First(x => x.Name == "Mike");
Student? student2 = list.FirstOrDefault(x => x.Name == "Mike");
Console.WriteLine(student1 == student2);
Console.WriteLine(student1);
```
# 4.获取最后一条数据
```java
Student student3 = list.Last(s => s.Score > 90);
Student? student4 = list.LastOrDefault(s => s.Score > 90);
Console.WriteLine(student3 == student4);
Console.WriteLine(student4);
```

# 5. 获取指定条数的数据Take
```java
Console.WriteLine("-----5.获取指定条数的数据------");
IEnumerable<Student> students1 = list.Take(3);
foreach (Student student in students1)
    { Console.WriteLine(student); }
Console.WriteLine("...");

IEnumerable<Student> students3 = list.Where(s=>s.Gender).Take(3);
foreach (Student student in students3)
    { Console.WriteLine(student); }
```
# 6.跳过指定条数的数据
```java
IEnumerable<Student> students4 = list.Skip(5);
foreach (Student student in students4)
    { Console.WriteLine(student); }
```
# 7.数据排序
```java
IEnumerable<Student> students5=list.Where(s=>s.Score>85).OrderBy(s=>s.Score);
IEnumerable<Student> students6 = list.Where(s => s.Score > 85).OrderByDescending(s => s.Score);
IEnumerable<Student> students7 = list.Where(s => s.Score > 85).OrderBy(s => s.Score).ThenByDescending(s => s.Gender);
```
# 8.运算
```java
int maxScore=list.Max(s=>s.Score); //最高分
int minScore=list.Min(s=>s.Score); //最低分
double avgScore=list.Average(s=>s.Score);
decimal sumSore=list.Sum(s=>s.Score);
int mScore = list.Where(s => s.Gender).Max(x => x.Score);    //男生最高分
double wScore=list.Where(s=>!s.Gender).Average(x=>x.Score); //女生平均分
```
# 9.分组
```java
IEnumerable<IGrouping<bool, Student>>? students8 = list.GroupBy(s=>s.Gender);
foreach (IGrouping<bool, Student> s in students8)
{
    bool gender = s.Key;
    int count=s.Count();
    double avgSocres=s.Average(s=>s.Score);
    //输出结果
    Console.WriteLine($"性别：{gender} 人数：{count} 平均值：{avgSocres}");
}
```
# 10.数据投影
```java
IEnumerable<string>? names=list.Select(s=>s.Name);
Console.WriteLine(string.Join(",\t", names));
IEnumerable<string>? sex= list.Select(s=>s.Gender?"男":"女");
Console.WriteLine(string.Join(",\t", sex));

var students9 = list.Select(s => new { s.Id,s.Name,Sex=s.Gender?"男":"女",s.Score});

foreach (var st in students9)
{
    Console.WriteLine(st);
}
```

# 11.结果转换
```java
Student[]? student11=list.Where(s => s.Score>85).ToArray();
List<Student>? student12 = list.Where(s => s.Score > 85).ToList();
Dictionary<int, Student>? student13 = list.Where(s => s.Score > 85).ToDictionary(s => s.Id);
```