Merge sorted `IEnumerable` (also works if they are unbounded)

MergeEnumerables.cs

    public static IEnumerable<T> Merge<T, TKey>(this IEnumerable<IEnumerable<T>> lists, Func<T, TKey> selector) where TKey : IComparable<TKey>
    {
        var enumerators = lists.Select(x => x.GetEnumerator()).ToList();
        try
        {
            enumerators.RemoveAll(x => !x.MoveNext());

            while (enumerators.Any())
            {
                var minEnumerator = enumerators.MinBy(x => selector(x.Current))!;

                yield return minEnumerator.Current;
                Debug.Assert(minEnumerator != null, nameof(minEnumerator) + " != null");
                if (!minEnumerator.MoveNext())
                {
                    enumerators.Remove(minEnumerator);
                }
            }
        }
        finally
        {
            foreach (var enumerator in enumerators)
            {
                enumerator.Dispose();
            }
        }
    }