IEnumerable extensions, Except on different collection type

IEnumerableExtension.cs

using System.Linq;

namespace System.Collections.Generic
{
	public static class IEnumerableExtension
	{
		public static IEnumerable<TFirst> Except<TFirst, TSecond, TCompared>(
			this IEnumerable<TFirst> first,
			IEnumerable<TSecond> second,
			Func<TFirst, TCompared> firstSelect,
			Func<TSecond, TCompared> secondSelect)
		{
			return Except(first, second, firstSelect, secondSelect, EqualityComparer<TCompared>.Default);
		}

		public static IEnumerable<TFirst> Except<TFirst, TSecond, TCompared>(
			this IEnumerable<TFirst> first,
			IEnumerable<TSecond> second,
			Func<TFirst, TCompared> firstSelect,
			Func<TSecond, TCompared> secondSelect,
			IEqualityComparer<TCompared> comparer)
		{
			if (first == null)
				throw new ArgumentNullException("first");
			if (second == null)
				throw new ArgumentNullException("second");
			return ExceptIterator(first, second, firstSelect, secondSelect, comparer);
		}

		private static IEnumerable<TFirst> ExceptIterator<TFirst, TSecond, TCompared>(
			IEnumerable<TFirst> first,
			IEnumerable<TSecond> second,
			Func<TFirst, TCompared> firstSelect,
			Func<TSecond, TCompared> secondSelect,
			IEqualityComparer<TCompared> comparer)
		{
			HashSet<TCompared> set = new HashSet<TCompared>(second.Select(secondSelect), comparer);
			foreach (TFirst tSource1 in first)
				if (set.Add(firstSelect(tSource1)))
					yield return tSource1;
		}
	}
}