IntersectWithHashset.cs

internal static IEnumerable<T> MyIntersect<T>(this IEnumerable<T> first, IEnumerable<T> second) {
   Debug.Assert(first != null);
   Debug.Assert(second != null);
   var firstHashset = first as HashSet<T>;
   var secondHashset = second as HashSet<T>;
   if (firstHashset != null && secondHashset != null) {
      return (firstHashset.Count > secondHashset.Count)
                ? firstHashset.Intersect(second)
                : secondHashset.Intersect(first);
   }
   if (firstHashset != null) { return firstHashset.Intersect(second); }
   if (secondHashset != null) { return secondHashset.Intersect(first); }
   return first.Intersect(second);
}

static IEnumerable<T> Intersect<T>(this HashSet<T> firstHashset, IEnumerable<T> second) {
   Debug.Assert(firstHashset != null);
   Debug.Assert(second != null);
   foreach (var tmp in second) {
      if (firstHashset.Contains(tmp)) { yield return tmp; }
   }
}