asp.net-mvc – 在ASP.NET MVC中未调用WebClient异步回调

public ActionResult Index(void) {
    webClient.DownloadStringComplete += onComplete;
    webClient.DownloadStringAsync(...);
    return null;
}

我发现直到Index()完成执行后才会调用onComplete.
我可以看到onComplete是在一个不同的线程上调用的,而是执行了一个索引.

问题：为什么会发生这种情况？为什么webClient的异步线程显然被阻塞,直到请求处理线程完成？

有没有办法解决这个问题,而无需从ThreadPool启动新线程(我试过这个,并且使用线程池确实按预期工作.如果从ThreadPool的线程调用DownloadStringAsync,webClient的回调确实按预期发生).

ASP.NET MVC 3.0,.NET 4.0,MS Cassini开发Web服务器(VS 2010)

编辑：这是一个完整的代码：

public class HomeController : Controller {
    private static ManualResetEvent done;

    public ActionResult Index() {
        return Content(DownloadString() ? "success" : "failure");
    }

    private static bool DownloadString() {
        try {
            done = new ManualResetEvent(false);
            var wc = new WebClient();
            wc.DownloadStringCompleted += (sender,args) => { 
                // this breakpoint is not hit until after Index() returns.
                // It is weird though,because response isn't returned to the client (browser) until this callback finishes.
                // Note: This thread is different from one Index() was running on.
                done.Set(); 
            };

            var uri = new Uri(@"http://us.battle.net/wow/en/character/blackrock/hunt/simple");

            wc.DownloadStringAsync(uri);

            var timedout = !done.WaitOne(3000);
            if (timedout) {
                wc.CancelAsync();
                // if this would be .WaitOne() instead then deadlock occurs.
                var timedout2 = !done.WaitOne(3000); 
                Console.WriteLine(timedout2);
                return !timedout2;
            }
            return true;
        }
        catch (Exception ex) {
            Console.WriteLine(ex.Message);
        }
        return false;
    }
}