If you just want simple parallelism, you can do this:
var bag = new ConcurrentBag<object>();
var tasks = myCollection.Select(async item =>
{
// some pre stuff
var response = await GetData(item);
bag.Add(response);
// some post stuff
});
await Task.WhenAll(tasks);
var count = bag.Count;
If you need something more complex, check out Stephen Toub’s ForEachAsync
post.