Find if listA contains any elements not in listB

Question

I have two lists:

List listA     
List listB

How to check using LINQ if in the listA exists an element w

Answer 1

You can do it in a single line

var res = listA.Where(n => !listB.Contains(n));

This is not the fastest way to do it: in case listB is relatively long, this should be faster:

var setB = new HashSet(listB);
var res = listA.Where(n => !setB.Contains(n));