A draft of enumerable merge sort method

EnumerableMergeSort.cs

private const int countThreshold = 64;
		
public static IEnumerable<TSource> MergeSort<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
	if (source == null)
		throw new ArgumentNullException("source");

	var countFull = source.Count();
	if (countFull < countThreshold)
		return source.OrderBy(keySelector);

	int countHalf = countFull / 2;

	var firstHalf = source.Take(countHalf).MergeSort(keySelector);
	var secondHalf = source.Skip(countHalf).MergeSort(keySelector);

	return firstHalf.Merge(secondHalf, keySelector);
}

public static IEnumerable<TSource> Merge<TSource, TKey>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, TKey> keySelector)
{
	if (first == null)
		throw new ArgumentNullException("first");
	if (second == null)
		throw new ArgumentNullException("second");

	var comparer = Comparer<TKey>.Default;

	using (var firstEnumerator = first.GetEnumerator())
	using (var secondEnumerator = second.GetEnumerator())
	{
		bool firstMoved = firstEnumerator.MoveNext();
		bool secondMoved = secondEnumerator.MoveNext();

		while (firstMoved && secondMoved)
		{
			if (comparer.Compare(keySelector(firstEnumerator.Current), keySelector(secondEnumerator.Current)) <= 0)
			{
				yield return firstEnumerator.Current;
				firstMoved = firstEnumerator.MoveNext();
			}
			else
			{
				yield return secondEnumerator.Current;
				secondMoved = secondEnumerator.MoveNext();
			}
		}

		while (firstMoved)
		{
			yield return firstEnumerator.Current;
			firstMoved = firstEnumerator.MoveNext();
		}

		while (secondMoved)
		{
			yield return secondEnumerator.Current;
			secondMoved = secondEnumerator.MoveNext();
		}
	}
}