LINQ 学习路程 -- 查询例子

IList<Student> studentList = new List<Student>() {
    new Student() { StudentID = , StudentName = "John", Age = , StandardID =  } ,
    new Student() { StudentID = , StudentName = "Steve",  Age = , StandardID =  } ,
    new Student() { StudentID = , StudentName = "Bill",  Age = , StandardID =  } ,
    new Student() { StudentID = , StudentName = "Ram" , Age = , StandardID =  } ,
    new Student() { StudentID = , StudentName = "Ron" , Age =  }
};

IList<Standard> standardList = new List<Standard>() {
    new Standard(){ StandardID = , StandardName="Standard 1"},
    new Standard(){ StandardID = , StandardName="Standard 2"},
    new Standard(){ StandardID = , StandardName="Standard 3"}
};

1.多个select和where操作

var studentNames = studentList.Where(s => s.Age > )
                              .Select(s => s)
                              .Where(st => st.StandardID > )
                              .Select(s => s.StudentName);

var teenStudentsName = from s in studentList
                       where s.age >  && s.age <
                       select new { StudentName = s.StudentName };

teenStudentsName.ToList().ForEach(s => Console.WriteLine(s.StudentName));

2.Group by

var studentsGroupByStandard = from s in studentList
                              group s by s.StandardID into sg
                              orderby sg.Key
                                    select new { sg.Key, sg };

foreach (var group in studentsGroupByStandard)
{
    Console.WriteLine("StandardID {0}:", group.Key);

    group.sg.ToList().ForEach(st => Console.WriteLine(st.StudentName ));
}

left outer join

var studentsGroup = from stad in standardList
                    join s in studentList
                    on stad.StandardID equals s.StandardID
                        into sg
                        select new {
                                        StandardName = stad.StandardName,
                                        Students = sg
                                    };

foreach (var group in studentsGroup)
{
    Console.WriteLine(group.StandardName);

    group.Students.ToList().ForEach(st => Console.WriteLine(st.StudentName));
}

var studentsWithStandard = from stad in standardList
                           join s in studentList
                           on stad.StandardID equals s.StandardID
                           into sg
                               from std_grp in sg
                               orderby stad.StandardName, std_grp.StudentName
                               select new {
                                                StudentName = std_grp.StudentName,
                                                StandardName = stad.StandardName
                                };

foreach (var group in studentsWithStandard)
{
    Console.WriteLine("{0} is in {1}", group.StudentName, group.StandardName);
}

Sorting

var sortedStudents = from s in studentList
                        orderby s.StandardID, s.age
                        select new {
                                StudentName = s.StudentName,
                                Age = s.age,
                                StandardID = s.StandardID };

sortedStudents.ToList().ForEach(s => Console.WriteLine("Student Name: {0}, Age: {1}, StandardID: {2}", s.StudentName, s.Age , s.StandardID));

inner join

var studentWithStandard = from s in studentList
                          join stad in standardList
                          on s.StandardID equals stad.StandardID
                          select new {
                                  StudentName = s.StudentName,
                                  StandardName = stad.StandardName
                              };

var nestedQueries = from s in studentList
                    where s.age >  && s.StandardID ==
                        (from std in standardList
                        where std.StandardName == "Standard 1"
                        select std.StandardID).FirstOrDefault()
                            select s;

nestedQueries.ToList().ForEach(s => Console.WriteLine(s.StudentName));