Parallel.ForEachAsync 不等待所有任務

Question

下面是示例控制台應用程序，output 是

Output 每次都不一樣，很好，但在我打印結果之前它需要完成所有任務。 似乎Parallel.ForEachAsync並沒有等待所有任務完成。 我在這里錯過了什么嗎？

internal class Program
{
    private async static Task Main(string[] args)
    {
        Stopwatch sw = new Stopwatch();
        sw.Start();
        await TestParallel();
        sw.Stop();
        Console.WriteLine("Elapsed={0}", sw.Elapsed);
        Console.ReadLine();
    }

    private static async Task TestParallel()
    {
        var tests = new List<int>() { 1, 2, 3, 4, 5, 6 };
        var options = new ParallelOptions { MaxDegreeOfParallelism = 5,
            CancellationToken = CancellationToken.None };
        var responses = new List<string>();
        await Parallel.ForEachAsync(tests, options, async (testno, cancellationToken) =>
        {
            var response = await TestTask(testno);
            responses.Add(response);
        });
        foreach (var response in responses)
        {
            Console.WriteLine(response);
        }
    }
    private static Task<string> TestTask(int testno)
    {
        System.Threading.Thread.Sleep(1000);
        return Task.FromResult($"Test{testno}");
    }
}

Answer 1

答案如下 - 更改行 var response = new ConcurrentBag();

internal class Program
{
    private async static Task Main(string[] args)
    {
        Stopwatch sw = new Stopwatch();
        sw.Start();
        await TestParallel();
        sw.Stop();
        Console.WriteLine("Elapsed={0}", sw.Elapsed);
        Console.ReadLine();
    }

    private static async Task TestParallel()
    {
        var tests = new List<int>() { 1, 2, 3, 4 ,5,6};
        var options = new ParallelOptions { MaxDegreeOfParallelism = 5, CancellationToken = CancellationToken.None };
        var responses = new ConcurrentBag<string>();
        await Parallel.ForEachAsync(tests, options, async (testno, cancellationToken) =>
        {
            var response = await TestTask(testno);
            responses.Add(response);
        });
        foreach (var response in responses)
        {
            Console.WriteLine(response);
        }
    }
    private static Task<string> TestTask(int testno)
    {
        System.Threading.Thread.Sleep(1000);
        return Task.FromResult($"Test{testno}");
    }
}

Answer 2

我認為您的示例有點令人困惑。 那是因為您使用了異步回調。 大多數異步用於 IO 目的。

go 用於：（這將受 CPU 限制，進行一些繁重的計算）

var responses = new List<string>();
var tests = new List<int>() { 1, 2, 3, 4 ,5,6};

await Parallel.ForEachAsync(tests, options, (testno, cancellationToken) =>
{
    // no async here...
    var response = TestTask(testno);
    // lock the shared resource.
    lock(responses)
        responses.Add(response);
});

foreach (var response in responses)
{
    Console.WriteLine(response);
}

private static string TestTask(int testno)
{
    // calculations done here
    System.Threading.Thread.Sleep(1000);
    return $"Test{testno}";
}

當您從 UI 線程啟動 CPU 密集型工作時，ForEachAsync 很有用。

或 go 用於：（這是 IO 綁定的，例如從外部來源獲取內容）

var tests = new List<int>() { 1, 2, 3, 4 ,5,6};

var tasks = new List<Task>();

// just add the tasks to a list, so you can await them later.
// the first part (till the first await) will be completed synchronous. 
// If any async/await is used, the Task.WhenAll will wait for it. 
// Multiple tasks can be running simultaneously.
foreach(var t in tests)
    tasks.Add(TestTask(t));

await Task.WhenAll(tasks);

foreach (var task in tasks)
{
    // the current thread won't be blocked by calling the .Result here
    // All tasks are already completed.
    Console.WriteLine(task.Result);
}

private static async Task<string> TestTask(int testno)
{
    // Getting information from external resources.
    await Task.Delay(1000);
    return $"Test{testno}";
}

（可能有一些錯字，沒有用VS寫）