Check if one IEnumerable contains all elements of another IEnumerable

There is no “fast way” to do this unless you track and maintain some state that determines whether all values in one collection are contained in another. If you only have IEnumerable<T> to work against, I would use Intersect.

var allOfList1IsInList2 = list1.Intersect(list2).Count() == list1.Count();

The performance of this should be very reasonable, since Intersect() will enumerate over each list just once. Also, the second call to Count() will be optimal if the underlying type is an ICollection<T> rather than just an IEnumerable<T>.