Find common items in multiple lists

Question

I searched, but I found only answers which related to two lists. But what about when they are more than two?

List 1 = 1,2,3,4,5
List 2 = 6,7,8,9,1
List 3 = 3,6,9,2,0,1
List 4 = 1,2,9,0,5
List 5 = 1,7,8,6,5,4
List 6 = 1
List 7 =

How to get the common items? as you can see one of them is empty, so the common will be empty, but I need to skip empty lists.

Answer 1

var data = new List<List<int>> {
    new List<int> {1, 2, 3, 4, 5},
    new List<int> {6, 7, 2, 8, 9, 1},
    new List<int> {3, 6, 9, 2, 0, 1},
    new List<int> {1, 2, 9, 0, 5},
    new List<int> {1, 7, 8, 6, 2, 5, 4},
    new List<int> {1, 7, 2}
};


List<int> res = data
    .Aggregate<IEnumerable<int>>((a, b) => a.Intersect(b))
    .ToList();

The type of Aggregate is explicitly given, otherwise aggregation of two Lists would have to be List too. It can be easily adapted to run in parallel:

List<int> res = data
    .AsParallel<IEnumerable<int>>()
    .Aggregate((a, b) => a.Intersect(b))
    .ToList();

EDIT

Except... it does not run in parallel. The problem is operations on IEnumerable are deferred, so even if they are logically merged in parallel context, the actual merging occurs in the ToList(), which is single threaded. For parallel execution it would be better to leave IEnumerable and return to the Lists:

List<int> res = data
    .AsParallel()
    .Aggregate((a, b) => a.Intersect(b).ToList());