WebException不应该发生在哪里?

var task = Task.Factory.StartNew(() =>
{
    while (!bc.IsCompleted && !cts.Token.IsCancellationRequested)
    {
        PriorityDownloadPair pd;
        if (bc.TryTake(out pd))
        {
            var baseUri = pd.Value.Uri;
            Console.WriteLine("({0}) {1}", pd.Key, baseUri.AbsoluteUri);
            IEnumerable<HtmlNode> sq = null;
            try
            {
                sq = SharpQuery.SharpQuery.Load(baseUri);
            }
            catch (WebException we)
            {
                Console.WriteLine(we.Message);
                continue;
            }
            foreach (var node in sq.Find("a[href]"))
            {
                bc.Add(new PriorityDownloadPair(1, new DownloadItem { Uri = new Uri(baseUri, node.Attributes["href"].Value) }));
            }
        }
    }
}, cts.Token);

它可以正常运行一段时间(跟踪并下载找到的每个链接),直到找到404。

public static IEnumerable<HtmlNode> Load(Uri uri)
{
    var doc = new HtmlDocument();
    WebClient wc = new WebClient();
    using (var str = wc.OpenRead(uri))
        doc.Load(str);
    yield return doc.DocumentNode;
}

如果我在调用堆栈上,它会指向这一行:

foreach (var node in sq.Find("a[href]"))

sq.Find 甚至不接触任何网络界面。发生什么事?

这些线是同步的,

        using (var str = wc.OpenRead(uri))
            doc.Load(str);