Why is my async ASP.NET Web API controller blocking the main thread?

Question

I have an ASP.NET Web API controller that I would have thought would operate asynchronously. The controller is designed to sleep 20 seconds for the first request, b

Answer 1

This actually has nothing to do with the server, and everything to do with the client. Both Chrome and Firefox don't appear to want to send what they deem a "duplicate" request until the first one has its response. A separate "private" session of either browser will return from the second request immediately. Internet Explorer 9 doesn't seem to exhibit this behaviour.

To isolate from client implementations, I put together the following client.

class Program
{
    static void Main(string[] args)
    {
        var t1 = Task.Run(() => FetchData(1));
        var t2 = Task.Run(() => FetchData(2));
        var t3 = Task.Run(() => FetchData(3));

        var index = Task.WaitAny(t1, t2, t3);
        Console.WriteLine("Task {0} finished first", index + 1);

        Task.WaitAll(t1, t2, t3);
        Console.WriteLine("All tasks have finished");

        Console.WriteLine("Press any key");
        Console.ReadKey(true);
    }

    static void FetchData(int clientNumber)
    {
        var client = new WebClient();
        string data = client.DownloadString("http://localhost:61852/api/values");
        Console.WriteLine("Client {0} got data: {1}", clientNumber, data);
    }
}

It's output goes:

1. Client 2 got data: "FOOBAR" (within milliseconds of startup)
2. Client 3 got data: "FOOBAR"
3. Task 2 finished first
4. (long wait here)
5. Client 1 got data: "FOOBAR"
6. All tasks have finished

Answer 2

In my case here is output (it switches to another thread from 5 to 10):

Entry thread id: 5. Sync: System.Web.LegacyAspNetSynchronizationContext
Sleepy thread id: 10. Sync: 
Finished sleeping
The thread '<No Name>' (0x2c84) has exited with code 0 (0x0).
Entry thread id: 7. Sync: System.Web.LegacyAspNetSynchronizationContext
The thread '<No Name>' (0x1818) has exited with code 0 (0x0).
Entry thread id: 5. Sync: System.Web.LegacyAspNetSynchronizationContext
The thread '<No Name>' (0xd4c) has exited with code 0 (0x0).
The thread '<No Name>' (0x2c30) has exited with code 0 (0x0).

This could be due to environment and machine running (single core) that makes the runtime decide how to run it.