C# async methods still hang UI

问题:

I have these two methods, that I want to run async to keep the UI responsive. However, it's still hanging the UI. Any suggestions?

async void DoScrape()
    {
        var feed = new Feed();

        var results = await feed.GetList();
        foreach (var itemObject in results)
        {
            var item = new ListViewItem(itemObject.Title);
            item.SubItems.Add(itemObject.Link);
            item.SubItems.Add(itemObject.Description);
            LstResults.Items.Add(item);
        }
    }


    public class Feed
    {
        async public Task<List<ItemObject>> GetList()
        {
            var client = new WebClient();
            string content = await client.DownloadStringTaskAsync(new Uri("anyUrl"));
            var lstItemObjects = new List<ItemObject>();
            var feed = new XmlDocument();
            feed.LoadXml(content);
            var nodes = feed.GetElementsByTagName("item");

            foreach (XmlNode node in nodes)
            {
                var tmpItemObject = new ItemObject();
                var title = node["title"];
                if (title != null) tmpItemObject.Title = title.InnerText;
                var link = node["link"];
                if (link != null) tmpItemObject.Link = link.InnerText;
                var description = node["description"];
                if (description != null) tmpItemObject.Description = description.InnerText;
                lstItemObjects.Add(tmpItemObject);
            }
            return lstItemObjects;
        }
    }

回答1:

I suspect DownloadStringTaskAsync relies upon HttpWebRequest.BeginGetResponse at a lower level. This being the case, it is known that the setup for a webrequest is not fully asynchronous. Annoyingly (and frankly, stupidly) the DNS lookup phase of an asynchronous WebRequest is performed synchronously, and therefore blocks. I suspect this might be the issue you are observing.

Reproduced below is a warning in the docs:

The BeginGetResponse method requires some synchronous setup tasks to complete (DNS resolution, proxy detection, and TCP socket connection, for example) before this method becomes asynchronous. As a result, this method should never be called on a user interface (UI) thread because it might take some time, typically several seconds. In some environments where the webproxy scripts are not configured properly, this can take 60 seconds or more. The default value for the downloadTime attribute on the config file element is one minute which accounts for most of the potential time delay.

You've two choices:

1. Start the request from a worker thread (and under high load, run the risk of ThreadPool starvation due to blocking behaviour)
2. (Tenuously) Perform a programmatic DNS lookup prior to firing the request. This can be done asynchronously. Hopefully the request will then use the cached DNS lookup.

We went for the 3rd (and costly) option of implementing our own properly asynchronous HTTP library to get decent throughput, but it's probably a bit extreme in your case ;)

回答2:

You seem to be confusing async with parallel. They are both based on Tasks, but they are completely different. Do not assume that async methods run in parallel -- they don't.

Async defaults to work in the same thread, unless there are reasons that force the async engine to spin up a new thread, such as the case when the main thread does not have a message pump. But in general, I tend to think of the async keyword as running in the same thread.

You use WinForms, so the UI thread has a message pump. Therefore, all your code above runs in the UI thread.

You must understand that you have NOT introduced any parallelism here. What you have introduced via the async keyword is asynchronous operations, NOT parallel. You have not done anything to "make your UI responsive" except for that one call to DownloadStringTaskAsync which won't force you to wait for the data to arrive, but you STILL have to do all the network processing (DNS lookup etc.) in the UI thread -- here is the asynchronous operation in play (you essentially "save" the time waiting for downloads).

In order to keep UI's responsive, you need to spin off time-consuming work into a separate thread while keeping the UI thread free. You're not doing this with the async keyword.

You need to use Task.Factory.StartNew(...) to explicitly spin up a new thread to do your background processing.

回答3:

How many items are you adding into your list view?

Unless you take action to prevent it, the WinForms list view will do a lot of processing every time you add an item into the list. This can take so long that adding just 100 items can take several seconds.

Try using BeginUpdate and EndUpdate around your loop to defer the bookkeeping of ListView until you're finished.

async void DoScrape()
{
    var feed = new Feed();

    var results = await feed.GetList();
    LstResults.BeginUpdate();  // Add this
    try
    {
        foreach (var itemObject in results)
        {
            var item = new ListViewItem(itemObject.Title);
            item.SubItems.Add(itemObject.Link);
            item.SubItems.Add(itemObject.Description);
            LstResults.Items.Add(item);
        }
    }
    finally
    {
        LstResults.EndUpdate();
    }
}

Got to use a try finally to avoid all sorts of pain if there's an exception.