IEnumerable

IEnumerable：可枚举类型，一个方法GetEnumerato()，实现该接口的类就可以对它进行遍历。
IEnumerator：枚举器，一个属性Current，两个方法MoveNext()，Reset()。

扩展方法

在不改变源码的基础上为类添加新的方法

namespace LinqTest
{
    public class Caculate
    {
        public int Add(int num1,int num2)
        {
            return num1 + num2;
        }
    }

    // 拓展Caculate类
    public static class CaculateExt
    {
        public static int AddExt(this Caculate caculate,int num1, int num2,int num3)
        {
            return num1 + num2 + num3;
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            Caculate caculate = new Caculate();
            int num1 = caculate.Add(1, 1);
            int num2 = caculate.AddExt(1, 1, 1);
            Console.WriteLine(num1);// 2
            Console.WriteLine(num2);// 3
        }
    }
}

Linq

linq to object 查询内存中的数据
linq to sql 查询数据库中的数据
linq to xml 查询xml文件

// 原始数据
List<Student> studentList = new List<Student>()
{
    new Student(){Id=001,Name="张三",Age=21,ClassId=1},
    new Student(){Id=002,Name="李四",Age=18,ClassId=1},
    new Student(){Id=003,Name="王五",Age=24,ClassId=2},
    new Student(){Id=004,Name="赵六",Age=16,ClassId=3},
    new Student(){Id=005,Name="陈小",Age=22,ClassId=3},
    new Student(){Id=006,Name="刘麻子",Age=28,ClassId=1}
};
List<StuClass> stuClasses = new List<StuClass>()
{
    new StuClass(){Id=1,ClassName="计科1班"},
    new StuClass(){Id=2,ClassName="计科2班"},
    new StuClass(){Id=3,ClassName="计科3班"}
};

Where 指定条件查询数据

1	List<Student> students1 = studentList.Where(stu => stu.Age > 18).ToList();

ForEach 遍历数据

1	students1.ForEach(stu => Console.WriteLine(stu));

Select 投影将原始类（一般为po）的属性投影到新的类（一般为dto）的属性中

1	var studentsWithIdAndName = studentList.Select(stu => new {Id=stu.Id,Name=stu.Name}).ToList();

Join 内连接

var studentsWithClass = studentList.Join(stuClasses,stu=>stu.ClassId,clazz =>clazz.Id,
                                         (stu,clazz)=>new StudentWithClass()
                                         { 
                                             Id=stu.Id,
                                             Name=stu.Name,
                                             Age=stu.Age,
                                             ClassId=stu.ClassId,
                                             ClassName=clazz.ClassName
                                         }).ToList();

 // 表达式写法 用于处理大于2个list的连接查询
var studentsWithClass2 = (from s in studentList join c in stuClasses on s.ClassId equals c.Id 
                          select new StudentWithClass()
                          {
                              Id = s.Id,
                              Name = s.Name,
                              Age = s.Age,
                              ClassId = s.ClassId,
                              ClassName = c.ClassName
                          }).ToList();

Take 获取指定数量的数据

Skip 跳过指定数量的数据

OrderBy 根据指定元素升序排列

OrderByDescending 根据指定元素降序排列

Where + Contains 模糊查询

1	var students2 = studentList.Where(stu => stu.Name.Contains("张")).OrderByDescending(stu => stu.Age).ToList();

GroupBy 指定属性进行分组

IEnumerable<IGrouping<int,Student>> items = studentList.GroupBy(s => s.Age);
foreach(IGrouping<int,Student> studentByAge in items)
{
    Console.WriteLine(studentByAge.Key);
    foreach(Student s in studentByAge)
    {
        Console.WriteLine(s);
    }
    Console.WriteLine("*************");
}