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所属分类：.NET技术
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 studentList where s.Age < 30 select 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 studentList where s.Age < 30 select 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 studentList where s.Age < 30 group s by s.ClassId into sg select 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 studentList join 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 studentList join c in classList on s.ClassId equals c.Id into scList from 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 } 
