intersect two lists with different objects

Question

I have a list of ObjA and ObjB as follows:

List List1;
List List2;

Both ObjA and ObjB has a common field which is U

Answer 1

without need of IEqualityComparer or IEquatable (which would be better anyway)

var commonUsers = list1
                  .Select(l1 => l1.User)
                  .Where(u => list1
                       .Select(l => l.User.Id)
                       .Intersect(list2
                          .Select(l2 => l2.Id))
                       .Contains(u.Id));

or

var commonUsers = list1.Select(l1 => l1.User)
                      .Where(u=> list2.Select(l2 => l2.User.Id)
                                        .Contains(u.Id));