Linq的学习

这里继续使用之前文章创建的学生类，首先简单介绍一下linq的使用。

Student.cs

 public class Student{public int Id { get; set; }public int ClassId { get; set; }public string Name { get; set; }public int Age { get; set; }public string Description { get; set; }public void Study(){Console.WriteLine($"{this.Id} {this.Name} 跟着老师学习 .Net开发");}public void StudyQt(){Console.WriteLine($"{this.Id} {this.Name} 跟着老师学习C++ Qt");}}

初始化学生数据

public class LinqPrinciple
{private List<Student> GetStudentsList(){List<Student> students = new List<Student>(){ new Student() { Id = 1, ClassId = 1, Name = "张三", Age = 20, Description = "张三是一个好学生" },new Student() { Id = 2, ClassId = 1, Name = "李四", Age = 21, Description = "李四是一个好学生" },new Student() { Id = 3, ClassId = 2, Name = "王五", Age = 22, Description = "王五是一个好学生" },new Student() { Id = 4, ClassId = 2, Name = "赵六", Age = 23, Description = "赵六是一个好学生" },new Student() { Id = 5, ClassId = 3, Name = "孙七", Age = 24, Description = "孙七是一个好学生" },new Student() { Id = 6, ClassId = 3, Name = "周八", Age = 25, Description = "周八是一个好学生" },new Student() { Id = 7, ClassId = 1, Name = "绿春", Age = 30, Description = "张三是一个好学生" },new Student() { Id = 8, ClassId = 1, Name = "麻醉", Age = 35, Description = "李四是一个好学生" },new Student() { Id = 9, ClassId = 2, Name = "开天", Age = 26, Description = "王五是一个好学生" },new Student() { Id = 10, ClassId = 2, Name = "匹敌", Age = 22, Description = "赵六是一个好学生" },new Student() { Id = 11, ClassId = 3, Name = "独轮车", Age = 23, Description = "孙七是一个好学生" },new Student() { Id = 12, ClassId = 3, Name = "火箭英", Age = 20, Description = "周八是一个好学生" }};return students;}
}

下面写几个简单的linq的语句，大概知道一下它的使用.

public void  Show(){//获取一下ClassID为3的学生们List<Student> students = GetStudentsList();List<Student> studentsId=students.Where(s => s.ClassId == 3).ToList();Console.WriteLine("********************************1*****************************");foreach (var student in studentsId){Console.WriteLine($"{student.Id} {student.Name} {student.Age} {student.Description}");}Console.WriteLine("********************************2*****************************");//获取一下年纪大于24的学生们List<Student> studentsAge = students.Where(s => s.Age > 24).ToList();foreach (var student in studentsAge){Console.WriteLine($"{student.Id} {student.Name} {student.Age} {student.Description}");}Console.WriteLine("********************************3*****************************");//获取一下年纪小于23同时ClassID为2的学生们List<Student> studentsAgeClassId = students.Where(s => s.Age < 23 && s.ClassId == 2).ToList();foreach (var student in studentsAgeClassId){Console.WriteLine($"{student.Id} {student.Name} {student.Age} {student.Description}");}}

linq的原理的探究

下面尝试探索一下这个linq的本质是什么?

尝试自己实现一下这3个需求代码,如下

 List<Student> students = GetStudentsList();List<Student> studentsId=new List<Student>();foreach (var student in students){if (student.ClassId == 3){studentsId.Add(student);}}List<Student> studentsAge=new List<Student>();foreach (var student in students){if (student.Age > 24){studentsAge.Add(student);}}List<Student> studentsAgeClassId=new List<Student>();foreach (var student in students){if (student.Age < 23 && student.ClassId == 2){studentsAgeClassId.Add(student);}}

这上面的代码有什么问题？

大量的重复代码---最好是封装一些，把重复的代码统一封装;

这里使用一个解决方案

封装一个方法，将判断的逻辑独立到一个小方法中去.

        public static bool  IsOk01(Student student){return student.ClassId == 3;}public static bool IsOk02(Student student){return student.Age > 24;}public static bool IsOk03(Student student){return student.Age < 23 && student.ClassId == 2;}public static List<Student> DragonWhere(List<Student> studentlist,Func<Student,bool> Func){List<Student> students = new List<Student>();foreach (var student in studentlist){if (Func(student)){students.Add(student);}}return students;}

尝试使用一下封装的函数

List<Student> students = GetStudentsList();
List<Student> studentId= MethodExtension.DragonWhere(students, MethodExtension.IsOk01);

这里还可以使用扩展方法和lambda表达式进一步的优化我们的代码

      public static List<Student> DragonWhere(this List<Student> studentlist,Func<Student,bool> Func){List<Student> students = new List<Student>();foreach (var student in studentlist){if (Func(student)){students.Add(student);}}return students;}

在调用的时候，

List<Student> students = GetStudentsList();
List<Student> studentsId = students.DragonWhere(s => s.ClassId == 3);

这里就串联上了我们之前学到的知识，最后为了支持多种数据类型，使用上泛型，最后我们就得到了最终封装的方法.

 public static List<T> DragonWhere<T>(this List<T> studentlist,Func<T,bool> Func){List<T> students = new List<T>();foreach (var student in studentlist){if (Func(student)){students.Add(student);}}return students;}

这里也要使用ilspy反编译一下它们的实现是如何？

看起来跟我们自己实现的Where差不多的样子.

再去看一下它的movenext函数.

这个就是一个yield return反汇编出来的状态机的代码。

linq常见的语句

 public void Show(){List<Student> studentList = this.GetStudentList(); #region Linq 扩展方法&表达式{  var list = studentList.Where<Student>(s => s.Age < 30); //list里面必然是符合要求的数据；foreach (var item in list){Console.WriteLine("Name={0}  Age={1}", item.Name, item.Age);}}{Console.WriteLine("********************");var list = from s in studentListwhere s.Age < 30select s;   //list里面必然是符合要求的数据；foreach (var item in list){Console.WriteLine("Name={0}  Age={1}", item.Name, item.Age);}}#endregion#region linq to object Show{Console.WriteLine("********************");var list = studentList.Where<Student>(s => s.Age < 30).Select(s => new  //投影：可以做一些自由组装+ new 一个匿名类，也可以new 具体类；{IdName = s.Id + s.Name,ClassName = s.ClassId == 2 ? "高级班" : "其他班"});foreach (var item in list){Console.WriteLine("Name={0}  Age={1}", item.ClassName, item.IdName);}}{Console.WriteLine("********************");var list = from s in studentListwhere s.Age < 30select new{IdName = s.Id + s.Name,ClassName = s.ClassId == 2 ? "高级班" : "其他班"};foreach (var item in list){Console.WriteLine("Name={0}  Age={1}", item.ClassName, item.IdName);}}{Console.WriteLine("********************");var list = studentList.Where<Student>(s => s.Age < 30)//条件过滤.Select(s => new//投影{Id = s.Id,ClassId = s.ClassId,IdName = s.Id + s.Name,ClassName = s.ClassId == 2 ? "高级班" : "其他班"}).OrderBy(s => s.Id)//排序 升序.ThenBy(s => s.ClassName) //多重排序，可以多个字段排序都生效.OrderByDescending(s => s.ClassId)//倒排.Skip(2)//跳过几条  //必须要先排序.Take(3)//获取几条 //必须要先排序;foreach (var item in list){Console.WriteLine($"Name={item.ClassName}  Age={item.IdName}");}}{//group by·Console.WriteLine("********************");var list = from s in studentListwhere s.Age < 30group s by s.ClassId into sgselect new{key = sg.Key,maxAge = sg.Max(t => t.Age)};foreach (var item in list){Console.WriteLine($"key={item.key}  maxAge={item.maxAge}");}//group by new {s.ClassId,s.Age}//group by new {A=s.ClassId>1}}{Console.WriteLine("********************");var list = studentList.GroupBy(s => s.ClassId).Select(sg => new{key = sg.Key,maxAge = sg.Max(t => t.Age)});foreach (var item in list){Console.WriteLine($"key={item.key}  maxAge={item.maxAge}");}}{var list = studentList.GroupBy(s => s.ClassId);foreach (var date in list) ///实现了IEnumerable{Console.WriteLine(date.Key);foreach (var item in date){Console.WriteLine(item.Age);}}}List<Class> classList = new List<Class>(){new Class(){Id=1,ClassName="架构班"},new Class(){Id=2,ClassName="高级班"},new Class(){Id=3,ClassName="全栈班"},};{//Join var list = from s in studentListjoin c in classList on s.ClassId equals c.Id  //只能使用equals  不能使==select new{Name = s.Name,CalssName = c.ClassName};foreach (var item in list){Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");}}{var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new{Name = s.Name,CalssName = c.ClassName});foreach (var item in list){Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");}}{//左连接var list = from s in studentListjoin c in classList on s.ClassId equals c.Idinto scListfrom sc in scList.DefaultIfEmpty()//select new{Name = s.Name,CalssName = sc == null ? "无班级" : sc.ClassName//c变sc，为空则用};foreach (var item in list){Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");}Console.WriteLine(list.Count());}{var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new{Name = s.Name,CalssName = c.ClassName}).DefaultIfEmpty();//为空就没有了foreach (var item in list){Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");}Console.WriteLine(list.Count());}{//左连接和右链接  就是链接对象交换一下即可；}#endregion}