任务陷入"WaitingForActivation“的状态

Question

这与.NET 4.0有关

我正在尝试使用Task异步地从多个WebAPI中获取数据。在控制台应用程序中，程序运行得很好，但是当我创建web应用程序时，任务的状态总是停留在"WaitingForActivation“上。任何帮助都将不胜感激。以下是我的代码：

    public String[] CallClientAPI(string[] addresses)
    {
        CancellationTokenSource cts = new CancellationTokenSource();
        int timeToWait = 3000; //wait for 3 seconds to fetch API Data

        Task.Factory.StartNew(() =>
        {
            Thread.Sleep(timeToWait);
            cts.Cancel();
        });

        Task<String[]> webTask = GetAPIData(addresses, cts.Token);
        //webTask is always in "WaitingForActivation" mode only ..

        if (!webTask.IsCompleted)
        {
            Thread.SpinWait(5000000);
        }

        String[] results = null;

        try
        {
            results = webTask.Result;
        }
        catch
        {               
        }
        //other code
    }

编辑：

    public Task<string[]> GetAPIData(String[] urls, CancellationToken token)
    {
        TaskCompletionSource<string[]> tcs = new TaskCompletionSource<string[]>();
        WebClient[] webClients = new WebClient[urls.Length];

        token.Register(() =>
            {
                foreach (var wc in webClients)
                {
                    if (wc != null)
                        wc.CancelAsync();
                }                    
            });

        object m_lock = new object();
        int count = 0;
        List<string> results = new List<string>();
        for (int i = 0; i < urls.Length; i++)
        {
            webClients[i] = new WebClient();

            webClients[i].DownloadStringCompleted += (obj, args) =>
                {
                    if (args.Cancelled == true)
                    {
                        tcs.TrySetCanceled();
                        return;
                    }
                    else if (args.Error != null)
                    {
                        tcs.TrySetException(args.Error);
                        return;
                    }
                    else
                    {
                        results.Add(args.Result);
                    }

                    lock (m_lock)
                    {
                        count++;
                        if (count == urls.Length)
                        {
                            tcs.TrySetResult(results.ToArray());
                        }
                    }
                };

            Uri address = null;
            try
            {
                address = new Uri(urls[i]);
                webClients[i].DownloadStringAsync(address, address);
            }

            catch (UriFormatException ex)
            {
                tcs.TrySetException(ex);
                return tcs.Task;
            }
        }

        return tcs.Task;
    }

Answer 1

这正是我们所期望的。简单又简单！！

    public String[] CallClientAPI(string[] addresses)
    {
        int timeToWait = Int32.Parse(ConfigurationManager.AppSettings["API_QUERY_TIMEOUT"].ToString());
        CancellationTokenSource cts = new CancellationTokenSource();

        Task.Factory.StartNew(() =>
            {
                Thread.Sleep(timeToWait);
                cts.Cancel();
            });

        ParallelOptions po = new ParallelOptions();
        po.CancellationToken = cts.Token;

        try
        {
            Parallel.ForEach(addresses, po, (address) =>
                {
                    try
                    {
                        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(address);
                        request.Method = "GET";
                        request.ContentType = "text/xml";
                        request.Timeout = timeToWait;

                        WebResponse response = request.GetResponse();

                        Stream respstream = response.GetResponseStream();
                        Encoding encode = System.Text.Encoding.GetEncoding("utf-8");
                        StreamReader sr = new StreamReader(respstream, encode);

                        myresult.Add(sr.ReadToEnd());
                    }
                    catch
                    {
                    }
                });
        }
        catch
        {
        }

        return myresult.ToArray();
    }

Answer 2

看起来您正在UI线程上开始下载。问题在于，Completed处理程序试图在UI线程上执行。但这是不可能的，因为UI线程在调用Result时被阻塞。其结果是您有一个死锁。

正确的解决方案是不阻止UI线程。这在C# 5.0中尤其有效(假设您使用的是VS 2012，可以与.Net 4.0一起使用)，但在C# 4.0中也可以使用ContinueWith()之类的东西。

如果您真的想在处理下载时阻止UI线程(我真的不建议这样做)，那么在一个单独的线程上执行所有的WebClient代码(使用Task.Factory.StartNew())。这样，Completed处理程序就不会试图在UI线程上恢复，从而解决死锁。