LINQ 学习路程 -- 查询操作 Select, SelectMany

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John" },
    new Student() { StudentID = 2, StudentName = "Moin" },
    new Student() { StudentID = 3, StudentName = "Bill" },
    new Student() { StudentID = 4, StudentName = "Ram" },
    new Student() { StudentID = 5, StudentName = "Ron" } 
};

var selectResult = from s in studentList
                   select s.StudentName; 

select可以定义结果数据类型，可以返回用户自定义的类型或者匿名类型

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 13 } ,
    new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
};

// returns collection of anonymous objects with Name and Age property
var selectResult = from s in studentList
                   select new { Name = "Mr. " + s.StudentName, Age = s.Age }; 

// iterate selectResult
foreach (var item in selectResult)
    Console.WriteLine("Student Name: {0}, Age: {1}", item.Name, item.Age);

方法查询

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 21 } 
};
    
var selectResult = studentList.Select(s => new { Name = s.StudentName , 
                                                 Age = s.Age  });

 SelectMany

 class Student
    {
        public int Score { get; set; }

        public Student(int score)
        {
            this.Score = score;
        }
    }

    class Teacher
    {
        public string Name { get; set; }

        public List<Student> Students;

        public Teacher(string order,List<Student> students)
        {
            this.Name = order;

            this.Students = students;
        }
    }

List<Teacher> teachers = new List<Teacher> 
            {
                new Teacher("a",new List<Student>{ new Student(100),new Student(90),new Student(30) }),
                new Teacher("b",new List<Student>{ new Student(100),new Student(90),new Student(60) }),
                new Teacher("c",new List<Student>{ new Student(100),new Student(90),new Student(40) }),
                new Teacher("d",new List<Student>{ new Student(100),new Student(90),new Student(60) }),
                new Teacher("e",new List<Student>{ new Student(100),new Student(90),new Student(50) }),
                new Teacher("f",new List<Student>{ new Student(100),new Student(90),new Student(60) }),
                new Teacher("g",new List<Student>{ new Student(100),new Student(90),new Student(60) })
            };

  这里有7个老师，每个人有3个学生，总共21一个学生里 查询3个倒霉蛋没考及格

var list1 = from t in teachers
            from s in t.Students
            where s.Score < 60
            select s;

var list2 = teachers.SelectMany(t => t.Students).Where(s => s.Score < 60);

public static IEnumerable<TResult> SelectMany<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TResult>> selector
)

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TCollection>> collectionSelector,
    Func<TSource, TCollection, TResult> resultSelector
)

选出了门下有不及格学生的倒霉蛋老师+门生的分数

var list3 = teachers.SelectMany(
                t => t.Students,
                (t, s) => new { t.Name, s.Score })
                .Where(n => n.Score < 60);