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();

Answer 1

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();