LINQ基础

 1 //这个代码声明了两个类
 2 //第一个代表学生,每个学生都有学生的名字和ID
 3 //第二个代表课程,每个课程类都有学生的ID和课程的名字
 4 class Program
 5 {
 6     public class Student
 7     {
 8         public int StID;
 9         public string LastName;
10     }
11     public class CourseStudent
12     {
13         public string CourseName;
14         public int StID;
15     }
16     //在全局中创建学生类与课程类
17     //课程类与学生类中有一样的ID
18     static Student[] students =newStudent[]{
19         new Student{StID=1,LastName="Carson"},
20         new Student{StID=2,LastName="Klassen"},
21         new Student{StID=3,LastName="Fleming"},
22     };
23     static CourseStudent[] studentsInCourses =new CourseStuden t[]{
24         new CourseStudent{CourseName="Art",StID=1},
25         new CourseStudent{CourseName="Art",StID=2},
26         new CourseStudent{CourseName="History",StID=1},
27         new CourseStudent{CourseName="History",StID=3},
28         new CourseStudent{CourseName="Physics",StID=3},
29     };
30     static void Main()
31     {
32         var query =from s in students//开始查询,在一个集合内查询
33                     join c in studentsInCourses on s.StID equals c.StID
34                     //而这个集合是,由两个集合的部分组成,上面的意思是:
35                     //定义students的迭代变量a
36                     //定义studentsInCourses迭代变量c
37                     //联结这连个集合中的成员,并生成新的一个集合
38                     //联结结果是:把两个集合成员具有相同StID的拿出来组成一个新的集合
39                     where c.CourseName=="History"
40                     //找出新集合中CourseName== "History"的成员
41                     select s.LastName;
42                     //把这些成员的LastName返回
43         foreach(var q in query)
44         Console.WriteLine("Student taking History: {0}", q);
45         Console.ReadKey();
46     }
47 }

 1 class Program
 2 {
 3     static void Main()
 4     {
 5         var groupA =new[]{3,4,5,6};
 6         var groupB =new[]{6,7,8,9};
 7         var someInts =from a in groupA//必须的第一个from语句
 8                     from b in groupB//主句中的from语句
 9                     select new{ a, b, sum=a+b };//创建一个匿名类型
10         foreach(var a in someInts )
11         Console.WriteLine( a );//很神奇,"a=","b="自动写了
12     }
13 }

 1 class Program
 2 {
 3     static void Main()
 4     {
 5         var groupA =new[]{3,4,5,6};
 6         var groupB =new[]{6,7,8,9};
 7         var someInts =from a in groupA
 8                     from b in groupB
 9                     let sum = a + b
10                     where sum ==12
11                     select new{ a, b, sum };
12         foreach(var a in someInts )
13         Console.WriteLine( a );
14     }
15 }

 1 static void Main()
 2 {
 3     var groupA =new[]{3,4,5,6};
 4     var groupB =new[]{6,7,8,9};
 5     var someInts =from int a in groupA
 6                 from int b in groupB
 7                 let sum = a + b
 8                 where sum >=11
 9                 where a ==4
10                 select new{ a, b, sum };
11     foreach(var a in someInts )
12     Console.WriteLine( a );
13 }

 1 classProgram
 2 {
 3     static void Main()
 4     {
 5         var students =new[]
 6         {
 7             new{LName="Jones",FName="Mary",Age=19,Major="History"},
 8             new{LName="Smith",FName="Bob",Age=20,Major="CompSci"},
 9             new{LName="Fleming",FName="Carol",Age=21,Major="History"}
10         };
11         var query =from student in students
12         orderby student.Age ascending//降序descending
13         select student;
14         foreach(var s in query)
15         {
16             Console.WriteLine("{0}, {1}: {2} - {3}",
17             s.LName, s.FName, s.Age, s.Major);
18         }
19         Console.ReadKey();
20     }
21 }

 1 classProgram
 2 {
 3     static void Main()
 4     {
 5         var students =new[]
 6         {
 7             new{LName="Jones",FName="Mary",Age=19,Major="History"},
 8             new{LName="Smith",FName="Bob",Age=20,Major="CompSci"},
 9             new{LName="Fleming",FName="Carol",Age=21,Major="History"}
10         };
11         var query =from student in students
12                    group student by student.Major;
13                    //query是返回的是IEnumerable<IGrouping<键,值>>,所以下面不能直接打印
14         foreach(var s in query )
15         {
16             Console.WriteLine("{0}", s.Key);//key是分组键
17             foreach(var t in s )
18             Console.WriteLine(" {0}, {1}", t.LName, t.FName);
19         }
20     }
21 }

 1 classProgram
 2 {
 3     static void Main()
 4     {
 5         var groupA =new[]{3,4,5,6};
 6         var groupB =new[]{4,5,6,7};
 7         var someInts =from a in groupA
 8                         join b in groupB on a equals b
 9                         into groupAandB
10                         from c in groupAandB
11                         //选中groupA与groupB相同的部分把这部分取名为grupAandB
12                         select c;
13         foreach(var a in someInts )
14         Console.Write("{0} ", a );
15     }
16 }