基本上，您需要为要上传的每个文件创建一个操作或任务，将它们放在一个列表中，然后处理该列表，从而限制可以并行处理的数量。
My blog post 展示了如何使用任务和操作来执行此操作，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效果。
有动作
如果使用 Actions，您可以使用内置的 .Net Parallel.Invoke 函数。这里我们限制它最多并行运行 4 个线程。
var listOfActions = new List<Action>();
foreach (var file in files)
{
var localFile = file;
// Note that we create the Task here, but do not start it.
listOfTasks.Add(new Task(() => UploadFile(localFile)));
}
var options = new ParallelOptions {MaxDegreeOfParallelism = 4};
Parallel.Invoke(options, listOfActions.ToArray());
这个选项虽然不支持异步，我假设你的 FileUpload 函数会，所以你可能想使用下面的任务示例。
有任务
Tasks 没有内置函数。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 async Task StartAndWaitAllThrottledAsync(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, CancellationToken cancellationToken = new CancellationToken())
{
await StartAndWaitAllThrottledAsync(tasksToRun, maxTasksToRunInParallel, -1, cancellationToken);
}
/// <summary>
/// Starts the given tasks and waits for them to complete. This will run the specified number of tasks in parallel.
/// <para>NOTE: If a timeout is reached before the Task completes, another Task may be started, potentially running more than the specified maximum allowed.</para>
/// <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 async Task StartAndWaitAllThrottledAsync(IEnumerable<Task> tasksToRun, int maxTasksToRunInParallel, int timeoutInMilliseconds, CancellationToken cancellationToken = new CancellationToken())
{
// Convert to a list of tasks so that we don'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.
await throttler.WaitAsync(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'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.
await Task.WhenAll(postTaskTasks.ToArray());
}
}
然后创建任务列表并调用函数让它们运行，例如一次最多同时运行 4 个，您可以这样做：
var listOfTasks = new List<Task>();
foreach (var file in files)
{
var localFile = file;
// Note that we create the Task here, but do not start it.
listOfTasks.Add(new Task(async () => await UploadFile(localFile)));
}
await Tasks.StartAndWaitAllThrottledAsync(listOfTasks, 4);
另外，由于此方法支持异步，它不会像使用 Parallel.Invoke 或 Parallel.ForEach 那样阻塞 UI 线程。