Task.WhenAll() - does it create a new thread?

Question

According to MSDN :

Creates a task that will complete when all of the supplied tasks have completed.

When Task.WhenAll() is called, it creates a task but does that necessarily mean that it creates a new thread to execute that task? For example, how many threads are created in this console application below?

class Program
{
    static void Main(string[] args)
    {
        RunAsync();
        Console.ReadKey();
    }

    public static async Task RunAsync()
    {

        Stopwatch sw = new Stopwatch();
        sw.Start();
        Task<string> google = GetString("http://www.google.com");
        Task<string> microsoft = GetString("http://www.microsoft.com");
        Task<string> lifehacker = GetString("http://www.lifehacker.com");
        Task<string> engadget = GetString("http://www.engadget.com");

        await Task.WhenAll(google, microsoft, lifehacker, engadget);
        sw.Stop();
        Console.WriteLine("Time elapsed: " + sw.Elapsed.TotalSeconds);
    }

    public static async Task<string> GetString(string url)
    {
        using (var client = new HttpClient())
        {
            return await client.GetStringAsync(url);
        }
    }
}

Answer 1

WhenAll does not create a new thread. A "task" does not necessarily imply a thread; there are two types of tasks: "event" tasks (eg, TaskCompletionSource ) and "code" tasks (eg, Task.Run ). WhenAll is an event-style task, so it does not represent code. If you're new to async , I recommend starting with my introductory blog post .

Your test application will use thread pool threads and IOCP threads as necessary to finish the async methods, so it may run with as few as 2 threads or as many as 5. If you're curious about how exactly the threading works, you can check out my recent blog post on async threads .