c# - 启动多个异步任务并在它们完成时处理它们 (C#)

Question

所以我正在尝试学习如何编写异步方法，并且一直在努力让异步调用工作。似乎总是发生的是代码卡在“等待”指令上，直到它最终似乎超时并以相同的方法使加载表单崩溃。

这很奇怪有两个主要原因:

1. 当非异步且只是一个简单的循环时，代码可以完美运行
2. 我几乎一字不差地复制了 MSDN 代码以将代码转换为此处的异步调用:https://msdn.microsoft.com/en-us/library/mt674889.aspx

我知道表格上已经有很多关于这个的问题，但我已经浏览了其中的大部分并尝试了很多其他方法(结果相同)，现在似乎认为在 MSDN 代码之后出现了根本性的错误没有工作。

这是后台 worker 调用的主要方法:

// this method loads data from each individual webPage
async Task LoadSymbolData(DoWorkEventArgs _e)
{
    int MAX_THREADS = 10;
    int tskCntrTtl = dataGridView1.Rows.Count;
    Dictionary<string, string> newData_d = new Dictionary<string, string>(tskCntrTtl);
    // we need to make copies of things that can change in a different thread
    List<string> links = new List<string>(dataGridView1.Rows.Cast<DataGridViewRow>()
        .Select(r => r.Cells[dbIndexs_s.url].Value.ToString()).ToList());
    List<string> symbols = new List<string>(dataGridView1.Rows.Cast<DataGridViewRow>()
        .Select(r => r.Cells[dbIndexs_s.symbol].Value.ToString()).ToList());
    // we need to create a cancelation token once this is working
    // TODO

    using (LoadScreen loadScreen = new LoadScreen("Querying stock servers..."))
    {
        // we cant use the delegate becaus of async keywords
        this.loaderScreens.Add(loadScreen);
        // wait until the form is loaded so we dont get exceptions when writing to controls on that form
        while ( !loadScreen.IsLoaded() );
        // load the total number of operations so we can simplify incrementing the progress bar
        // on seperate form instances
        loadScreen.LoadProgressCntr(0, tskCntrTtl);
        // try to run all async tasks since they are non-blocking threaded operations
        for (int i = 0; i < tskCntrTtl; i += MAX_THREADS)
        {
            List<Task<string[]>> ProcessURL = new List<Task<string[]>>();
            List<int> taskList = new List<int>();

            // Make a list of task indexs
            for (int task = i; task < i + MAX_THREADS && task < tskCntrTtl; task++)
                taskList.Add(task);

            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<string[]>> downloadTasksQuery =
                from task in taskList select QueryHtml(loadScreen, links[task], symbols[task]);

            // ***Use ToList to execute the query and start the tasks. 
            List<Task<string[]>> downloadTasks = downloadTasksQuery.ToList();

            // ***Add a loop to process the tasks one at a time until none remain.
            while (downloadTasks.Count > 0)
            {
                // Identify the first task that completes.
                Task<string[]> firstFinishedTask = await Task.WhenAny(downloadTasks);   // <---- CODE HANGS HERE

                // ***Remove the selected task from the list so that you don't
                // process it more than once.
                downloadTasks.Remove(firstFinishedTask);

                // Await the completed task.
                string[] data = await firstFinishedTask;
                if (!newData_d.ContainsKey(data.First())) 
                    newData_d.Add(data.First(), data.Last());
            }
        }
        // now we have the dictionary with all the information gathered from teh websites
        // now we can add the columns if they dont already exist and load the information
        // TODO
        loadScreen.UpdateProgress(100);
        this.loaderScreens.Remove(loadScreen);
    }
}

下面是查询网页的异步方法:

async Task<string[]> QueryHtml(LoadScreen _loadScreen, string _link, string _symbol) 
{
    string data = String.Empty;

    try
    {
        HttpClient client = new HttpClient();
        var doc = new HtmlAgilityPack.HtmlDocument();
        var html = await client.GetStringAsync(_link);    // <---- CODE HANGS HERE
        doc.LoadHtml(html);

        string percGrn = doc.FindInnerHtml(
            "//span[contains(@class,'time_rtq_content') and contains(@class,'up_g')]//span[2]");
        string percRed = doc.FindInnerHtml(
            "//span[contains(@class,'time_rtq_content') and contains(@class,'down_r')]//span[2]");

        // create somthing we'll nuderstand later
        if ((String.IsNullOrEmpty(percGrn) && String.IsNullOrEmpty(percRed)) ||
            (!String.IsNullOrEmpty(percGrn) && !String.IsNullOrEmpty(percRed)))
            throw new Exception();

        // adding string to empty gives string
        string perc = percGrn + percRed;
        bool isNegative = String.IsNullOrEmpty(percGrn);
        double percDouble;

        if (double.TryParse(Regex.Match(perc, @"\d+([.])?(\d+)?").Value, out percDouble))
            data = (isNegative ? 0 - percDouble : percDouble).ToString();
    }
    catch (Exception ex) { }
    finally
    {
        // update the progress bar...
        _loadScreen.IncProgressCntr();
    }

    return new string[] { _symbol, data };
}

我真的需要一些帮助。谢谢!

Answer 1

简而言之，当您将异步与任何“常规”任务函数结合使用时，您会遇到死锁

http://olitee.com/2015/01/c-async-await-common-deadlock-scenario/

解决方案是使用configureawait

var html = await client.GetStringAsync(_link).ConfigureAwait(false);

你需要这个的原因是因为你没有等待你的原始线程。

// ***Create a query that, when executed, returns a collection of tasks.
IEnumerable<Task<string[]>> downloadTasksQuery = from task in taskList select QueryHtml(loadScreen,links[task], symbols[task]);

这里发生的事情是您将等待范例与三个常规任务处理范例混合在一起。并且那些不会混合(或者更确切地说，您必须使用 ConfigureAwait(false) 才能工作。