C# Linq语句用法大全以及Lambda表达式

C# Linq语句用法大全以及Lambda表达式

Linq： 是一种用于数据查询和操作的语言集成查询（Language Integrated Query）技术。通过 Linq，我们可以使用类似于 SQL 查询的方式来查询、筛选和操作各种类型的数据集合，包括数组、列表、集合、XML 文档、数据库表等等。
常见的有：
Linq to Objects：用于操作对象集合，例如数组、列表等。
Linq to XML：用于操作 XML 数据，支持类似于 XPath 的查询。
Linq to SQL：用于操作关系型数据库，以及进行 SQL 查询。
Linq to Entities：用于操作实体框架数据模型。
Linq to DataSet：用于操作 ADO.NET DataSet 数据集合。

Lambda： Lambda 表达式是一种简洁、方便的语法形式，用于定义匿名函数。它通常用于需要传递一个函数作为参数的场景。

文章主要讲解 Linq 语句的用法

数据准备

新建一个ClassStudent类
属性：ID，班级名称

internal class ClassStudent
{
   public int Id { get; set; }
   public string ClassName { get; set; }
}

初始化ClassStudent

List<ClassStudent> classStudentList = new List<ClassStudent>();
classStudentList.Add(new ClassStudent() { Id = 1, ClassName = "1班" });
classStudentList.Add(new ClassStudent() { Id = 2, ClassName = "2班" });
classStudentList.Add(new ClassStudent() { Id = 3, ClassName = "3班" });

新建一个Student的类
属性：ID，姓名，年龄，ClassStudent.ID

internal class Student
{
   public int Id { get; set; }
   public string Name { get; set; }
   public int Age { get; set; }
}   

初始化Student

List<Student> studentList = new List<Student>();
studentList.Add(new Student() { Id = 1, Name = "妲己", Age = 18, ClassID = 1 });
studentList.Add(new Student() { Id = 2, Name = "申公豹", Age = 28, ClassID = 1 });
studentList.Add(new Student() { Id = 3, Name = "纣王", Age = 38, ClassID = 1 });
studentList.Add(new Student() { Id = 4, Name = "姬发", Age = 20, ClassID = 2 });
studentList.Add(new Student() { Id = 5, Name = "姜子牙", Age = 78, ClassID = 2 });
studentList.Add(new Student() { Id = 6, Name = "哪吒", Age = 56, ClassID = 2 });
studentList.Add(new Student() { Id = 7, Name = "杨戬", Age = 23, ClassID = 3 });
studentList.Add(new Student() { Id = 8, Name = "孙悟空", Age = 1002, ClassID = 3 });

Selcet的用法

查询所有的学生

//Linq语句
var data=from n in studentList select n;
//Lambda表达式
var data = studentList.Select(n => n);
foreach (var student in data) {
    Console.WriteLine($"姓名：{student.Name},年龄：{student.Age}");
}

查询所有的学生的姓名

//Linq语句
var data = from student in students
           select new { student.Name, student.Age };
//Lambda表达式
var data = students.Select(student => new { student.Name, student.Age });
foreach (var student in data) {
    Console.WriteLine($"姓名：{student}");
}

查询年龄大于18岁的学生

//Linq语句
var data = from n in studentList where n.Age > 18 select n;
//Lambda表达式
var data = studentList.Where(n => n.Age > 18);
foreach (var student in data) {
    Console.WriteLine($"姓名：{student.Name},年龄：{student.Age}");
}

根据年龄升序

//Linq语句
var data = from n in studentList orderby n.Age ascending select n;
//Lambda表达式
var data = studentList.OrderBy(n => n.Age);
foreach (var student in data) {
    Console.WriteLine($"姓名：{student.Name},年龄：{student.Age}");
}

根据年龄降序

//Linq语句
var data = from n in studentList orderby n.Age descending select n;
//Lambda表达式
var data = studentList.OrderByDescending(n => n.Age);
foreach (var student in data) {
    Console.WriteLine($"姓名：{student.Name},年龄：{student.Age}");
}

连接查询

//Linq语句
var data = from n in studentList
		   join g in classStudentList on
		   n.ClassID equals g.Id
		   where g.ClassName == "1班"
		   select new { n.Name, g.ClassName };
//Lambda表达式
var data = studentList.Join(
           classStudentList,
           n => n.ClassID,
           g => g.Id,
           (n, g) => new { n.Name, g.ClassName }
           ).Where(x => x.ClassName == "1班");
foreach (var student in data)
{
   Console.WriteLine($"姓名：{student.Name},班级：{student.ClassName}");
}

弱连接查询

//Linq语句
var data = from n in studentList
           join g in classStudentList on n.ClassID equals g.Id
           into x
           from g in x.DefaultIfEmpty()
           select new
           {
              Name = n.Name,
              ClassName = g==null ? "" : g.ClassName
           };
//Lambda表达式
var data = studentList.GroupJoin(
           classStudentList,
           n => n.ClassID,
           g => g.Id,
           (n, group) => new { n, group }
           ).SelectMany(
           x => x.group.DefaultIfEmpty(),
           (x, g) => new
           {
             Name = x.n.Name,
             ClassName = g == null ? "" : g.ClassName
           }
           );
foreach (var student in data)
{
   Console.WriteLine($"姓名：{student.Name},班级：{student.ClassName}");
}

分组用法

//Linq语句
var data = from n in studentList
           group n by n.ClassID into g
           select new
           {
              ClassID = g.Key,
              Age = g.Sum(x => x.Age)
           };
//Lambda表达式
var data = studentList.GroupBy(
           n => n.ClassID,
           (key, group) => new
           {
              ClassID = key,
              Age = group.Sum(x => x.Age)
            }
           );
foreach (var student in data)
{
   Console.WriteLine($"ClassID：{student.ClassID},年龄：
}

分组查询，根据连接classStudentList的ID，查询classStudentList的数量

//linq语句
var data = from n in studentList
            join c in classStudentList on n.ClassID equals c.Id
            into groups
            orderby groups.Count()
            select new
            {
               studentList = n,
               count = groups.Count(),
            };
//Lambda表达式
var data = studentList
            .Join(classStudentList,
                  n => n.ClassID,
                  c => c.Id,
                  (n, c) => new { n, c })
            .GroupBy(x => x.n)
            .Select(g => new 
            { 
                studentList = g.Key, 
                count = g.Count() 
            })
            .OrderBy(x => x.count);
foreach (var student in data)
{
   Console.WriteLine($"姓名：{student.studentList.Name},数量：{student.count }");
}