ExecuteInParallel.cs

    public static class TplHelpers
    {
        public static async Task<List<T2>> ExecuteInParallel<T1,T2>(this IEnumerable<T1> collection,
            Func<T1, Task<T2>> processor,
            int degreeOfParallelism)
        {
            var queue = new ConcurrentQueue<T1>(collection);
            var result = new ConcurrentBag<T2>();
            var tasks = Enumerable.Range(0, degreeOfParallelism).Select(async _ =>
            {
                while (queue.TryDequeue(out T1 item))
                {
                    var r = await processor(item);
                    result.Add(r);
                }
            });

            await Task.WhenAll(tasks);
            return result.ToList();
        }
    }