My name
is
Jon Skeet

How to group list of objects, which contains another list?

I have a class with these properties:

public string FirstName { get; set; }
public string LastName { get; set; }
public int ID { get; set; }
public GradeLevel Year;
public List<int> ExamScores;

And a method:

protected static List<Student> students = new List<Student>
{
    new Student {FirstName = "Terry", LastName = "Adams", ID = 120, 
        Year = GradeLevel.SecondYear, 
        ExamScores = new List<int>{ 99, 82, 81, 92}},
    new Student {FirstName = "Fadi", LastName = "Fakhouri", ID = 116, 
        Year = GradeLevel.ThirdYear,
        ExamScores = new List<int>{ 99, 81, 90, 94}},
    new Student {FirstName = "Hanying", LastName = "Feng", ID = 117, 
        Year = GradeLevel.FirstYear, 
        ExamScores = new List<int>{ 90, 94, 81, 82}},
}

Next step:

public void GroupByMark()
{
    students.ForEach(mark =>
    {
        var queryNumericRange =
            from student in students
            group new {student.FirstName, student.LastName} by mark
            into percentGroup
            orderby percentGroup.Key
            select percentGroup;
        foreach (var studentGroup in queryNumericRange)
        {
            Console.WriteLine("Key: {0}", (studentGroup.Key));
            foreach (var item in studentGroup)
            {
                Console.WriteLine("\t{0}, {1}", item.LastName, item.FirstName);
            }
        }
    });
}

Result:

key: test.StudentClass+Student
    Adams, Terry
    Fakhouri, Fadi
    Feng, Hanying

key: test.StudentClass+Student
    Adams, Terry
    Fakhouri, Fadi
    Feng, Hanying

key: test.StudentClass+Student
    Adams, Terry
    Fakhouri, Fadi
    Feng, Hanying

Desired result:

key: 99
    Adams, Terry
    Fakhouri, Fadi

key: 82
    Adams, Terry
    Feng, Hanying

key: 81
    Adams, Terry
    Fakhouri, Fadi

key: 92
    Adams, Terry

key: 90
    Fakhouri, Fadi
    Feng, Hanying

key: 94
    Fakhouri, Fadi
    Feng, Hanying

So, for each mark I would like to display list of students, who received that particular mark. Do you have any ideas how to do it?

It sounds like you want to create a sequence of student/mark pairs, and then group by the mark:

var studentsByMark = from student in students
                     from mark in student.ExamScores
                     group student by mark;
foreach (var group in studentsByMark)
{
    Console.WriteLine("{0}:", group.Key); // Mark
    foreach (var student in group)
    {
        Console.WriteLine("{0}, {1}", student.LastName, student.FirstName);
    }
}

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