对于一个更简单的解决方案(不确定是否是最优的)，您可以简单地嵌套Parallel。ForEach在Task中-就像这样

var options = new ParallelOptions { MaxDegreeOfParallelism = 5 }
Task.Run(() =>
{
    Parallel.ForEach(myCollection, options, item =>
    {
        DoWork(item);
    }
}

ParallelOptions将为您进行开箱即用的节流。

我在一个真实的场景中使用它在后台运行一个很长的操作。这些操作是通过HTTP调用的，它的设计目的是在运行长操作时不阻塞HTTP调用。

调用HTTP进行长时间后台操作。 操作从后台开始。 用户获得状态ID，可用于使用另一个HTTP调用检查状态。 后台操作更新其状态。

这样，CI/CD调用就不会因为长时间的HTTP操作而超时，而是每隔x秒循环一次状态，而不会阻塞进程

使用SemaphoreSlim可以实现并行控制。

var bag = new ConcurrentBag<object>();
var maxParallel = 20;
var throttler = new SemaphoreSlim(initialCount: maxParallel);
var tasks = myCollection.Select(async item =>
{
  await throttler.WaitAsync();
  try
  {
     var response = await GetData(item);
     bag.Add(response);
  }
  finally
  {
     throttler.Release();
  }
});
await Task.WhenAll(tasks);
var count = bag.Count;

在接受的应答中，ConcurrentBag不是必需的。 下面是一个没有它的实现:

var tasks = myCollection.Select(GetData).ToList();
await Task.WhenAll(tasks);
var results = tasks.Select(t => t.Result);

任何"// some pre stuff"和"// some post stuff"都可以进入GetData实现(或另一个调用GetData的方法)

除了更短之外，没有使用“async void”lambda，这是一种反模式。

从其他答案和接受的asnwer引用的文章编译的最简单的可能扩展方法:

public static async Task ParallelForEachAsync<T>(this IEnumerable<T> source, Func<T, Task> asyncAction, int maxDegreeOfParallelism)
{
    var throttler = new SemaphoreSlim(initialCount: maxDegreeOfParallelism);
    var tasks = source.Select(async item =>
    {
        await throttler.WaitAsync();
        try
        {
            await asyncAction(item).ConfigureAwait(false);
        }
        finally
        {
            throttler.Release();
        }
    });
    await Task.WhenAll(tasks);
}

更新:这是一个简单的修改，也支持取消令牌，就像在评论中要求的(未经测试)

public static async Task ParallelForEachAsync<T>(this IEnumerable<T> source, Func<T, CancellationToken, Task> asyncAction, int maxDegreeOfParallelism, CancellationToken cancellationToken)
{
    var throttler = new SemaphoreSlim(initialCount: maxDegreeOfParallelism);
    var tasks = source.Select(async item =>
    {
        await throttler.WaitAsync(cancellationToken);
        if (cancellationToken.IsCancellationRequested) return;

        try
        {
            await asyncAction(item, cancellationToken).ConfigureAwait(false);
        }
        finally
        {
            throttler.Release();
        }
    });
    await Task.WhenAll(tasks);
}

我的ParallelForEach异步的轻量级实现。

特点:

节流(最大并行度)。 异常处理(聚合异常将在完成时抛出)。 内存效率高(不需要存储任务列表)。

public static class AsyncEx
{
    public static async Task ParallelForEachAsync<T>(this IEnumerable<T> source, Func<T, Task> asyncAction, int maxDegreeOfParallelism = 10)
    {
        var semaphoreSlim = new SemaphoreSlim(maxDegreeOfParallelism);
        var tcs = new TaskCompletionSource<object>();
        var exceptions = new ConcurrentBag<Exception>();
        bool addingCompleted = false;

        foreach (T item in source)
        {
            await semaphoreSlim.WaitAsync();
            asyncAction(item).ContinueWith(t =>
            {
                semaphoreSlim.Release();

                if (t.Exception != null)
                {
                    exceptions.Add(t.Exception);
                }

                if (Volatile.Read(ref addingCompleted) && semaphoreSlim.CurrentCount == maxDegreeOfParallelism)
                {
                    tcs.TrySetResult(null);
                }
            });
        }

        Volatile.Write(ref addingCompleted, true);
        await tcs.Task;
        if (exceptions.Count > 0)
        {
            throw new AggregateException(exceptions);
        }
    }
}

使用的例子:

await Enumerable.Range(1, 10000).ParallelForEachAsync(async (i) =>
{
    var data = await GetData(i);
}, maxDegreeOfParallelism: 100);

我为此创建了一个扩展方法，它使用SemaphoreSlim，还允许设置最大并行度

    /// <summary>
    /// Concurrently Executes async actions for each item of <see cref="IEnumerable<typeparamref name="T"/>
    /// </summary>
    /// <typeparam name="T">Type of IEnumerable</typeparam>
    /// <param name="enumerable">instance of <see cref="IEnumerable<typeparamref name="T"/>"/></param>
    /// <param name="action">an async <see cref="Action" /> to execute</param>
    /// <param name="maxDegreeOfParallelism">Optional, An integer that represents the maximum degree of parallelism,
    /// Must be grater than 0</param>
    /// <returns>A Task representing an async operation</returns>
    /// <exception cref="ArgumentOutOfRangeException">If the maxActionsToRunInParallel is less than 1</exception>
    public static async Task ForEachAsyncConcurrent<T>(
        this IEnumerable<T> enumerable,
        Func<T, Task> action,
        int? maxDegreeOfParallelism = null)
    {
        if (maxDegreeOfParallelism.HasValue)
        {
            using (var semaphoreSlim = new SemaphoreSlim(
                maxDegreeOfParallelism.Value, maxDegreeOfParallelism.Value))
            {
                var tasksWithThrottler = new List<Task>();

                foreach (var item in enumerable)
                {
                    // Increment the number of currently running tasks and wait if they are more than limit.
                    await semaphoreSlim.WaitAsync();

                    tasksWithThrottler.Add(Task.Run(async () =>
                    {
                        await action(item).ContinueWith(res =>
                        {
                            // action is completed, so decrement the number of currently running tasks
                            semaphoreSlim.Release();
                        });
                    }));
                }

                // Wait for all tasks to complete.
                await Task.WhenAll(tasksWithThrottler.ToArray());
            }
        }
        else
        {
            await Task.WhenAll(enumerable.Select(item => action(item)));
        }
    }

示例用法:

await enumerable.ForEachAsyncConcurrent(
    async item =>
    {
        await SomeAsyncMethod(item);
    },
    5);