linq 用法和实践

一 linq 用法

方法语法和查询语法

在使用LINQ写查询时可以使用两种形式的语法：查询语法和方法语法

　　a:方法语法（method syntax） ： 使用标准的方法调用，这些方法是一组叫做标准查询运算符的方法

　　b:查询语法（query method） ： 看上去和SQL语句很相似，使用查询表达式形式书写。微软推荐使用查询语法，因为它更易读

在编译时，CLR会将查询语法转换为方法语法

int[] num = { 2, 4, 6, 8, 10 };

var numQuery = from number in num //查询语法
where number < 8
select number;

var numMethod = num.Where(x => x < 8); //方法语法

它们得到的结果是一样的。方法语法中where的参数使用了Lambda表达式

https://blog.csdn.net/qq_38916538/article/details/107359698

https://www.cnblogs.com/liqingwen/p/5801249.html

https://www.cnblogs.com/liqingwen/p/5832322.htmls

https://blog.csdn.net/zlbcdn/article/details/79192485

https://blog.csdn.net/lym940928/article/details/80278783
这篇文章很好
https://www.nhooo.com/linq/linq-lambda-expression.html

二实践

   public class Student
    {
        public int stId { get; set; }
        public string stuName { get; set; }

        public int num;
    }
    public class Course
    {
        public int stId; //学生ID
        public string courseName; //课程名
        public int chengji;
        public int num2;
    }

    var query2 = from s in students
                         join c in courses on s.stId equals c.stId
                         select new { s.stId, s.stuName, c.courseName, c.chengji };

实例一

 var  query3 = query2.Select(x=>x.courseName);
  foreach (var item in query3)
             {
                Console.WriteLine(item);
            //     //输出
            //     //art
            //     //history
            //     //art
            //     //history
            //     //physics
            //     //art
            //     //history
            //     //art
            //     //history
            //     //physics
            }

实例二

  foreach (var item in query2)
            {
       Console.WriteLine(item);
  }

输出

{ stId = 1, stuName = jack, courseName = art, chengji = 80 }
{ stId = 1, stuName = jack, courseName = history, chengji = 70 }
{ stId = 2, stuName = taylor, courseName = art, chengji = 88 }
{ stId = 3, stuName = fleming, courseName = history, chengji = 60 }
{ stId = 3, stuName = fleming, courseName = physics, chengji = 100 }

实例三

var query4 = query2.GroupBy(x => x.courseName, (index, item) =>
 {  //在这里 index取值是 x.courseName
     return item;//.Where(m => m.chengji > 81);
  }).ToList();
 foreach (var item in query4)
    {
               Console.WriteLine(item.FirstOrDefault()+" ======ddd");
    }

输出

{ stId = 1, stuName = jack, courseName = art, chengji = 80 } ======ddd
{ stId = 1, stuName = jack, courseName = history, chengji = 70 } ======ddd
{ stId = 3, stuName = fleming, courseName = physics, chengji = 100 } ======ddd

实例四

 var query5 = query2.GroupBy(x => x.courseName, (indexGroupKey, item) =>
            {  //indexGroupKey 取值是  x.courseName
                
                   // return new { indexGroupKey, newData= item.FirstOrDefault()}; 这也是一种方法
                var newItem = item.FirstOrDefault();//取第一项
                return new { fenzuziduan= indexGroupKey, stId= newItem.stId, courseName=newItem.courseName, stuName=newItem.stuName,sumData=item.Sum(fen=>fen.chengji) };
            }).ToList();
 foreach (var item in query5)
            {
                Console.WriteLine(item);
            }

输出

{ fenzuziduan = art, stId = 1, courseName = art, stuName = jack, sumData = 168 }
{ fenzuziduan = history, stId = 1, courseName = history, stuName = jack, sumData = 130 }
{ fenzuziduan = physics, stId = 3, courseName = physics, stuName = fleming, sumData = 100 }

运行结果图

    public class StudentBig
    {
        public int StudentID { get; set; }
        public string StudentName { get; set; }
        public int Age { get; set; }
    }
     IList<StudentBig> studentList = new List<StudentBig>() {
        new StudentBig() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new StudentBig() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
        new StudentBig() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new StudentBig() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new StudentBig() { StudentID = 5, StudentName = "Abram" , Age = 21 }
    };

            var groupedResult = studentList.GroupBy(s => s.Age);

            foreach (var ageGroup in groupedResult)
            {
                Console.WriteLine("Age Group: {0}", ageGroup.Key);  //每组都有一个钥匙 

                foreach (StudentBig s in ageGroup)  //每个组都有一个内部集合  
                    Console.WriteLine("Student Name: {0}", s.StudentName);
            }