异步控制器行为

AsyncManager.Parameters["headlines"] = value;正在分配标题值。但是当调用NewsCompleted方法时，headlines变为空。

public void NewsAsync(string city)
    {
        AsyncManager.OutstandingOperations.Increment();
        NewsService newsService = new NewsService();
        newsService.GetHeadlinesCompleted += (value) =>
        {
            AsyncManager.Parameters["headlines"] = value;
            AsyncManager.OutstandingOperations.Decrement();
        };
        newsService.GetHeadlinesAsync(city);
    }
    public ActionResult NewsCompleted(string[] headlines)
    {
        return View("News", new ViewStringModel
        {
            NewsHeadlines = headlines
        });
    }

1.）如何调试？

2.）并且当OutstandingOperations计数变为0时，框架通过从线程池请求新线程来调用我的actionCompleted方法。当这个actionCompleted方法被调用时，我查看了调用堆栈，不知道后面到底发生了什么。我的意思是，框架如何知道它应该用AsyncManager中的这个参数调用这个特定的actionCompleted方法？

谢谢你的帮助。

public void NewsAsync(string city)
{
    AsyncManager.OutstandingOperations.Increment();
    NewsService newsService = new NewsService();
    newsService.GetHeadlinesCompleted += (sender, args) =>
    {
        AsyncManager.Parameters["headlines"] = args.Result;
        AsyncManager.OutstandingOperations.Decrement();
    };
    newsService.GetHeadlinesAsync(city);
}
public ActionResult NewsCompleted(object headlines)
{
    return View("News", new ViewStringModel
    {
        NewsHeadlines = (string[])headlines
    });
}