Run async method 8 times in parallel

Question

How do I turn the following into a Parallel.ForEach?

public async void getThreadContents(String[] threads)
{
    HttpClient client = new HttpClient();
    Li         


        

Answer 1

Stephen Toub has a good blog post on implementing a ForEachAsync. Svick's answer is quite good for platforms on which Dataflow is available.

Here's an alternative, using the partitioner from the TPL:

public static Task ForEachAsync(this IEnumerable source,
    int degreeOfParallelism, Func body)
{
  var partitions = Partitioner.Create(source).GetPartitions(degreeOfParallelism);
  var tasks = partitions.Select(async partition =>
  {
    using (partition) 
      while (partition.MoveNext()) 
        await body(partition.Current); 
  });
  return Task.WhenAll(tasks);
}

You can then use this as such:

public async Task getThreadContentsAsync(String[] threads)
{
  HttpClient client = new HttpClient();
  ConcurrentDictionary usernames = new ConcurrentDictionary();

  await threads.ForEachAsync(8, async url =>
  {
    HttpResponseMessage response = await client.GetAsync(url);
    String content = await response.Content.ReadAsStringAsync();
    String user;
    foreach (Match match in regex.Matches(content))
    {
      user = match.Groups[1].ToString();
      usernames.TryAdd(user, null);
    }
    progressBar1.PerformStep();
  });
}