select List where sub-list contains is all item from another list with linq

Question

I have a model with list items:

public class Student{
    public int StudentId { get; set; }
    public int ClassId { get; set; }
}

The table values are similar to the following:

StudentId	ClassId
1	8
2	6
1	3
3	8
2	3
3	2
4	8
1	6
3	6
2	2

classId list for filter:

ClassId
8
6

I want to select the list of StudentId where are in all filter classId.

StudentId
1
3

I use this code but not working:

List<int> lstStudentId = Students.GroupBy(o => o.StudentId).Where(o => o.All(m => filterClassId.All(s => s == m.ClassId ))).Select(o => o.Key).ToList();

Ok got you. You can use Intersect: `var eight = new int[]{1,3,4}; var six = new int[]{2,1,3}; var result = eight.Intersect(six); foreach(var r in result) Console.WriteLine(r);` — mshwf, Apr 07 '23 at 10:51
@vernou: I tested similar to this and did not get an answer. Please give a more complete code — misha co, Apr 07 '23 at 10:54
@mshwf: Please answer according to my question codes. I don't understand the usage of your code according to my question — misha co, Apr 07 '23 at 11:02
Maybe similar to : https://stackoverflow.com/questions/1520642/does-net-have-a-way-to-check-if-list-a-contains-all-items-in-list-b — vernou, Apr 07 '23 at 11:14

score 2 · Accepted Answer · answered Apr 07 '23 at 11:11

You write :

List<int> lstStudentId = Students
    .GroupBy(o => o.StudentId)
    .Where(o => o.All(m => filterClassId.All(s => s == m.ClassId)))
    .Select(o => o.Key).ToList();

The Where check all student's classes are in the filter... but you want the inverse, all filtered classes are in the student.

Just reverse the condition :

var lstStudentId = Students
    .GroupBy(o => o.StudentId)
    .Where(g => filterClassId.All(f => g.Any(s => s.ClassId == f)))
    .Select(g => g.Key)
    .ToList();

A alternative, it's to check if the filter less the student's class result in nothing. Then the student has all class in the filter :

var lstStudentId = Students
    .GroupBy(o => o.StudentId)
    .Where(g => !filterClassId.Except(g.Select(s => s.ClassId)).Any())
    .Select(g => g.Key)
    .ToList();

score 0 · Answer 2 · answered Apr 07 '23 at 11:27

You can use Intersect, first implemet IEqualityComparer for the Student model:

class StudentComparer : IEqualityComparer<Student>
{
    public bool Equals(Student? x, Student? y) => x.StudenId == y.StudenId;
    public int GetHashCode([DisallowNull] Student obj) => obj.StudenId.GetHashCode();
}

Then you are able to use Intersect as follows:

var commonStudents = students.Where(x => x.ClassId == 8)
    .Intersect(students.Where(x => x.ClassId == 6), new StudentComparer())
    .Select(x => new { x.StudenId }).ToList();

select List where sub-list contains is all item from another list with linq

2 Answers2