我可以限制同时运行的System.Threading.Tasks.Task对象的数量吗？

Question

我正在使用Task类来进行多线程计算.

例如:

   List<Task> taskList = new List<Task>();
   for(int i=0;i<10;i++){
        var task = new Task(() => Console.WriteLine("Hello from taskA."));
        taskList.Add(task);
        task.Start();
    }

有没有办法让我们说,最多只能运行3个任务,其余的要等待？

Answer 1

我的博客文章展示了如何使用"任务"和"操作"执行此操作,并提供了一个示例项目,您可以下载并运行以查看两者的实际操作.

有了动作

如果使用Actions,则可以使用内置的.Net Parallel.Invoke函数.在这里,我们将其限制为最多并行运行3个线程.

var listOfActions = new List<Action>();
for (int i = 0; i < 10; i++)
{
    // Note that we create the Action here, but do not start it.
    listOfActions.Add(() => DoSomething());
}

var options = new ParallelOptions {MaxDegreeOfParallelism = 3};
Parallel.Invoke(options, listOfActions.ToArray());

随着任务

由于您在这里使用任务,因此没有内置功能.但是,您可以使用我在博客上提供的那个.

    /// <summary>
    /// Starts the given tasks and waits for them to complete. This will run, at most, the specified number of tasks in parallel.
    /// <para>NOTE: If one of the given tasks has already been started, an exception will be thrown.</para>
    /// </summary>
    /// <param name="tasksToRun">The tasks to run.</param>
    /// <param name="maxTasksToRunInParallel">The maximum number of tasks to run in parallel.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    public static void StartAndWaitAllThrottled(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, CancellationToken cancellationToken = new CancellationToken())
    {
        StartAndWaitAllThrottled(tasksToRun, maxTasksToRunInParallel, -1, cancellationToken);
    }

    /// <summary>
    /// Starts the given tasks and waits for them to complete. This will run, at most, the specified number of tasks in parallel.
    /// <para>NOTE: If one of the given tasks has already been started, an exception will be thrown.</para>
    /// </summary>
    /// <param name="tasksToRun">The tasks to run.</param>
    /// <param name="maxTasksToRunInParallel">The maximum number of tasks to run in parallel.</param>
    /// <param name="timeoutInMilliseconds">The maximum milliseconds we should allow the max tasks to run in parallel before allowing another task to start. Specify -1 to wait indefinitely.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    public static void StartAndWaitAllThrottled(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, int timeoutInMilliseconds, CancellationToken cancellationToken = new CancellationToken())
    {
        // Convert to a list of tasks so that we don&#39;t enumerate over it multiple times needlessly.
        var tasks = tasksToRun.ToList();

        using (var throttler = new SemaphoreSlim(maxTasksToRunInParallel))
        {
            var postTaskTasks = new List<Task>();

            // Have each task notify the throttler when it completes so that it decrements the number of tasks currently running.
            tasks.ForEach(t => postTaskTasks.Add(t.ContinueWith(tsk => throttler.Release())));

            // Start running each task.
            foreach (var task in tasks)
            {
                // Increment the number of tasks currently running and wait if too many are running.
                throttler.Wait(timeoutInMilliseconds, cancellationToken);

                cancellationToken.ThrowIfCancellationRequested();
                task.Start();
            }

            // Wait for all of the provided tasks to complete.
            // We wait on the list of "post" tasks instead of the original tasks, otherwise there is a potential race condition where the throttler&#39;s using block is exited before some Tasks have had their "post" action completed, which references the throttler, resulting in an exception due to accessing a disposed object.
            Task.WaitAll(postTaskTasks.ToArray(), cancellationToken);
        }
    }

然后创建任务列表并调用函数让它们运行,一次最多同时执行3次,你可以这样做:

var listOfTasks = new List<Task>();
for (int i = 0; i < 10; i++)
{
    var count = i;
    // Note that we create the Task here, but do not start it.
    listOfTasks.Add(new Task(() => Something()));
}
Tasks.StartAndWaitAllThrottled(listOfTasks, 3);

Answer 2

更改MaxDegreeOfParallelism属性。

例子