How to correctly write Parallel.For with async methods

Question

How would I structure the code below so that the async method gets invoked?

Parallel.For(0, elevations.Count(), delegate(int i)
{
   allSheets.AddRange(await         


        

Answer 1

You can try this code I'm using. it using foreach and SemaphoreSlim to achive parallel asynchronous.

public static class ParallelAsync
{
    public static async Task ForeachAsync(IEnumerable source, int maxParallelCount, Func action)
    {
        using (SemaphoreSlim completeSemphoreSlim = new SemaphoreSlim(1))
        using (SemaphoreSlim taskCountLimitsemaphoreSlim = new SemaphoreSlim(maxParallelCount))
        {
            await completeSemphoreSlim.WaitAsync();
            int runningtaskCount = source.Count();

            foreach (var item in source)
            {
                await taskCountLimitsemaphoreSlim.WaitAsync();

                Task.Run(async () =>
                {
                    try
                    {
                        await action(item).ContinueWith(task =>
                        {
                            Interlocked.Decrement(ref runningtaskCount);
                            if (runningtaskCount == 0)
                            {
                                completeSemphoreSlim.Release();
                            }
                        });
                    }
                    finally
                    {
                        taskCountLimitsemaphoreSlim.Release();
                    }
                }).GetHashCode();
            }

            await completeSemphoreSlim.WaitAsync();
        }
    }
}

usage:

string[] a = new string[] {
    "1",
    "2",
    "3",
    "4",
    "5",
    "6",
    "7",
    "8",
    "9",
    "10",
    "11",
    "12",
    "13",
    "14",
    "15",
    "16",
    "17",
    "18",
    "19",
    "20"
};

Random random = new Random();

await ParallelAsync.ForeachAsync(a, 2, async item =>
{
    Console.WriteLine(item + " start");

    await Task.Delay(random.Next(1500, 3000));
    Console.WriteLine(item + " end");
});

Console.WriteLine("All finished");

any suggestion please let me know.