如何在不运行它们的情况下合并两个Linq IEnumerable <T>查询？

Question

如何合并List<T>基于TPL的任务以便以后执行？

 public async IEnumerable<Task<string>> CreateTasks(){ /* stuff*/ }

我的假设是.Concat()......

     void MainTestApp()  // Full sample available upon request.
     {
        List<string> nothingList = new List<string>();
        nothingList.Add("whatever");
        cts = new CancellationTokenSource();

         delayedExecution =
            from str in nothingList
            select AccessTheWebAsync("", cts.Token);
         delayedExecution2 =
          from str in nothingList
          select AccessTheWebAsync("1", cts.Token);

         delayedExecution = delayedExecution.Concat(delayedExecution2);
     }


    /// SNIP

    async Task AccessTheWebAsync(string nothing, CancellationToken ct)
    {
        // return a Task
    }

我想确保这不会产生任何任务或评估任何东西.事实上,我想我在问"逻辑上执行IQueryable到返回数据的东西"？

背景

由于我正在进行递归,并且我不想在正确的时间之前执行此操作,如果多次调用,合并结果的正确方法是什么？

如果重要,我正在考虑运行此命令来启动var AllRunningDataTasks = results.ToList();此代码后面的所有任务:

while (AllRunningDataTasks.Count > 0)
{
    // Identify the first task that completes.
    Task<TableResult> firstFinishedTask = await Task.WhenAny(AllRunningDataTasks);

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

    // TODO: Await the completed task.
    var taskOfTableResult = await firstFinishedTask;

    // Todo: (doen't work)
    TrustState thisState = (TrustState)firstFinishedTask.AsyncState;

    // TODO: Update the concurrent dictionary with data
    // thisState.QueryStartPoint + thisState.ThingToSearchFor 

    Interlocked.Decrement(ref thisState.RunningDirectQueries);
    Interlocked.Increment(ref thisState.CompletedDirectQueries);

    if (thisState.RunningDirectQueries == 0)
    {
        thisState.TimeCompleted = DateTime.UtcNow;
    }
}

Answer 1

以下是一种合并数据的黑客方法...我不喜欢在 Main 或此示例的其他一些方面必须使用“nothingList”，但它似乎完成了工作并允许我合并待处理的任务。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add a using directive and a reference for System.Net.Http.
using System.Net.Http;

// Add the following using directive.
using System.Threading;


namespace ProcessTasksAsTheyFinish
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;
        List<IEnumerable<Task>> launchList = new List<IEnumerable<Task>>();

        public MainWindow()
        {
            InitializeComponent();

            List<string> nothingList = new List<string>();
            nothingList.Add("whatever");

            cts = new CancellationTokenSource();

             delayedExecution =
                from str in nothingList
                select AccessTheWebAsync("", cts.Token);


             List<string> nothingList2 = new List<string>();
             nothingList2.Add("whatever");

             delayedExecution2 =
              from str in nothingList2
              select AccessTheWebAsync("1", cts.Token);


             launchList.Add(delayedExecution);
             launchList.Add(delayedExecution2);

             delayedExecution = delayedExecution.Concat(delayedExecution2);
        }
        IEnumerable<Task> delayedExecution = null;
        IEnumerable<Task> delayedExecution2 = null;

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();

            // Instantiate the CancellationTokenSource.

            try
            {
                // ***Set up the CancellationTokenSource to cancel after 25 seconds.
                //cts.CancelAfter(250000);

                var test  =  delayedExecution;// AccessTheWebAsync("", cts.Token);

                var testList = test.ToList();

                while (testList.Count() > 0)
                {
                    var firstFinishedTask = await Task.WhenAny(testList);
                    testList.Remove(firstFinishedTask);

                      await firstFinishedTask;
                }

                resultsTextBox.Text += "\r\nDownloads complete.";
            }
            catch (OperationCanceledException tee)
            {
                resultsTextBox.Text += "\r\nDownloads canceled.\r\n";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownloads failed.\r\n";
            }

            cts = null;
        }


        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        async Task<string> AccessTheWebAsync(string nothing, CancellationToken ct)
        {
            // CHANGE THIS VALUE TO CONTROL THE TESTING
            bool delayConversionOfQueryToList = false;

            HttpClient client = new HttpClient();

            // Make a list of web addresses.
            List<string> urlList = null;

            if (nothing == "1")
            {
                urlList = SetUpURLList2();
            }
            else urlList = SetUpURLList();

            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<int>> downloadTasksQuery =
                from url in urlList select ProcessURL(url, client, ct);

            // DEBUG!!!
            if (delayConversionOfQueryToList == true)
            {
                await Task.Delay(10000);
                resultsTextBox.Text += String.Format("\r\nDelay of IQueryable complete.  Tip: Did you see any IsRunning messages?");
            }

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

            // DEBUG!!!
            if (delayConversionOfQueryToList == false)
            {
                await Task.Delay(10000);
                resultsTextBox.Text += String.Format("\r\nDelay of .ToList() complete.   Tip: Did you see any IsRunning messages?");
            }

            // ***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<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

                resultsTextBox.Text += String.Format("\r\nID  {0}", firstFinishedTask.Id);

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

                // Await the completed task.
                int length = await firstFinishedTask;
                resultsTextBox.Text += String.Format("\r\nLength of the download:  {0}", length);
            }

            return nothing;
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "http://msdn.microsoft.com",
                "http://msdn.microsoft.com/library/windows/apps/br211380.aspx",
                "http://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "http://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "http://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "http://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "http://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }
        private List<string> SetUpURLList2()
        {
            List<string> urls = new List<string> 
            { 
                "http://www.google.com",

            };
            return urls;
        }

        async Task<int> ProcessURL(string url, HttpClient client, CancellationToken ct)
        {
            resultsTextBox.Text += String.Format("\r\nIS RUNNING {0}", url);

            // GetAsync returns a Task<HttpResponseMessage>. 
            HttpResponseMessage response = await client.GetAsync(url, ct);
            // Retrieve the website contents from the HttpResponseMessage.
            byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

           // Thread.Sleep(3000);
           // await Task.Delay(1000, ct);
           return urlContents.Length;
        }
    }
}

// Sample Output:

IS RUNNING http://msdn.microsoft.com
IS RUNNING http://msdn.microsoft.com/library/windows/apps/br211380.aspx
IS RUNNING http://msdn.microsoft.com/en-us/library/hh290136.aspx
IS RUNNING http://msdn.microsoft.com/en-us/library/dd470362.aspx
IS RUNNING http://msdn.microsoft.com/en-us/library/aa578028.aspx
IS RUNNING http://msdn.microsoft.com/en-us/library/ms404677.aspx
IS RUNNING http://msdn.microsoft.com/en-us/library/ff730837.aspx
IS RUNNING http://www.google.com
Delay of .ToList() complete.   Tip: Did you see any IsRunning messages?
ID  1
Length of the download:  48933
ID  2
Length of the download:  375328
ID  3
Length of the download:  220428
ID  4
Length of the download:  222256
ID  5
Length of the download:  229330
ID  6
Length of the download:  136544
ID  7
Length of the download:  207171
Delay of .ToList() complete.   Tip: Did you see any IsRunning messages?
ID  8
Length of the download:  43945
Downloads complete.