extension to append an array in parallel

InParallel.cs

        public static IEnumerable<TAggregate> InParallel<TSource, TAggregate>(
            this IEnumerable<TSource> source,
            Func<TSource, Task<IEnumerable<TAggregate>>> parallelTask) =>
            InParallel(source, parallelTask, k => k);

        public static IEnumerable<TAggregate> InParallel<TSource, TAggregate, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, Task<IEnumerable<TAggregate>>> parallelTask,
            Func<TSource,TKey> keySelector)
        {
            var concurrentDictionary = new ConcurrentDictionary<TKey, IEnumerable<TAggregate>>();
            var enumerable = source
                .Select(async i =>
                {
                    var pResult = await parallelTask(i);
                    var key = keySelector(i);
                    concurrentDictionary.TryAdd(key, pResult);
                });
            Task.WaitAll(enumerable.ToArray());

            var sortedDictionary = new SortedDictionary<TKey, IEnumerable<TAggregate>>();
            foreach (var keyValuePair in concurrentDictionary)
            {
                sortedDictionary.Add(keyValuePair.Key, keyValuePair.Value);
            }

            return sortedDictionary.SelectMany(kp => kp.Value);
        }

append a task in parallel