c# - 如何检查此间隔是否正在运行？

Question

我已经设置了一堆 Console.WriteLine，据我所知，当我在 .NET Fiddle 中运行以下命令时，它们都没有被调用。

using System;
using System.Net;
using System.Linq.Expressions;
using System.Linq;  
using System.Threading;
using System.Threading.Tasks;
using System.Timers;
using System.Collections.Generic;

public class Program
{
    private static readonly object locker = new object();
    private static readonly string pageFormat = "http://www.letsrun.com/forum/forum.php?board=1&page={0}";

    public static void Main()
    {
        var client = new WebClient();

        // Queue up the requests we are going to make
        var tasks = new Queue<Task<string>>(
            Enumerable
            .Repeat(0,50)
            .Select(i => new Task<string>(() => client.DownloadString(string.Format(pageFormat,i))))
        );

        // Create set of 5 tasks which will be the at most 5
        // requests we wait on
        var runningTasks = new HashSet<Task<string>>();
        for(int i = 0; i < 5; ++i)
        {
            runningTasks.Add(tasks.Dequeue());
        }

        var timer = new System.Timers.Timer
        {
            AutoReset = true,
            Interval = 2000 
        };

        // On each tick, go through the tasks that are supposed
        // to have started running and if they have completed
        // without error then store their result and run the
        // next queued task if there is one. When we run out of 
        // any more tasks to run or wait for, stop the ticks.
        timer.Elapsed += delegate
        {
            lock(locker)
            {
                foreach(var task in runningTasks)
                {
                    if(task.IsCompleted)
                    {
                        if(!task.IsFaulted)
                        {
                            Console.WriteLine("Got a document: {0}", 
                                task.Result.Substring(Math.Min(30, task.Result.Length)));

                            runningTasks.Remove(task);

                            if(tasks.Any())
                            {
                                runningTasks.Add(tasks.Dequeue());
                            }
                        }
                        else
                        {
                            Console.WriteLine("Uh-oh, task faulted, apparently");
                        }
                    }
                    else if(!task.Status.Equals(TaskStatus.Running)) // task not started
                    {
                        Console.WriteLine("About to start a task.");
                        task.Start();
                    }
                    else
                    {
                        Console.WriteLine("Apparently a task is running.");
                    }
                }   

                if(!runningTasks.Any())
                {
                    timer.Stop();
                }
            }

        };
    }
}

我也非常感谢有关如何简化或修复其中任何错误逻辑的建议。我正在尝试做的模式就像

(1) 创建一个有N个任务的queueu

(2)创建一组M个任务，从(1)中的前M个出队项

(3)启动M个任务运行

(4) X 秒后，检查已完成的任务。

(5) 对于任何已完成的任务，对结果做一些事情，从集合中删除任务并用队列中的另一个任务替换它(如果队列中还有剩余的话)。

(6) 无限重复(4)-(5)。

(7) 如果集合没有剩余任务，我们就完成了。

但也许有更好的方法来实现它，或者也许有一些 .NET 函数可以轻松封装我正在尝试做的事情(与指定的最大并行度并行的 Web 请求)。

Answer 1

您的代码中有几个问题，但由于您正在寻找更好的方法来实现它 - 您可以使用 Parallel.For 或 Parallel.ForEach:

Parallel.For(0, 50, new ParallelOptions() { MaxDegreeOfParallelism = 5 }, (i) =>
{
     // surround with try-catch
     string result;
     using (var client = new WebClient()) {
          result = client.DownloadString(string.Format(pageFormat, i));
     }
     // do something with result
     Console.WriteLine("Got a document: {0}", result.Substring(Math.Min(30, result.Length)));
});

它将并行执行正文(在任何给定时间不超过 5 个任务)。当一项任务完成后 - 下一项任务开始，直到它们全部完成，就像您想要的那样。

更新。使用这种方法有几个等待来限制任务，但最直接的就是 sleep :

Parallel.For(0, 50, new ParallelOptions() { MaxDegreeOfParallelism = 5 },  
(i) =>
{
    // surround with try-catch
    var watch = Stopwatch.StartNew();
    string result;
    using (var client = new WebClient()) {
         result = client.DownloadString(string.Format(pageFormat, i));
    }
    // do something with result
    Console.WriteLine("Got a document: {0}", result.Substring(Math.Min(30, result.Length)));
    watch.Stop();
    var sleep = 2000 - watch.ElapsedMilliseconds;
    if (sleep > 0)
          Thread.Sleep((int)sleep);
});