c＃中的並行與任務

Question

我有一個方法，它需要進行多個服務調用並合並結果。 試圖使用Task這個並且結果不對，所以我做了以下兩個測試。

使用任務

List<Task<string>> tasks = new List<Task<string>>();
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for (int i = 0; i < 20; i++)
{
    tasks.Add(Task.Factory.StartNew(() =>
        {
            long started = stopwatch.ElapsedMilliseconds;
            Random wait = new Random();
            int waited = wait.Next(500, 3000);
            Thread.Sleep(waited);
            return string.Format("Index #{0} started at {1}ms and waited {2}ms", i, started, waited);
        }));
}

Task.WaitAll(tasks.ToArray());
foreach (Task<string> task in tasks)
{
    Console.WriteLine(string.Format("[Task {0,2}] {1}", task.Id, task.Result));
}

使用並行

Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
int[] inputs = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 };
Parallel.ForEach(inputs, i =>
{
    long started = stopwatch.ElapsedMilliseconds;
    Random wait = new Random();
    int waited = wait.Next(500, 3000);
    Thread.Sleep(waited);
    string result = string.Format("Task {0,2} started at {1} ms and waited {2} ms", i, started, waited);
    Console.WriteLine(result);
});

stopwatch.Stop();
Console.WriteLine("Total time taken: " + stopwatch.ElapsedMilliseconds.ToString());

如下所示，任務索引始終打印為20（雖然傳遞了0到19）。 此外，Parallel比任務花費更多時間。 以下是結果，顯然我在任務中做錯了，無法弄清楚:(

任務輸出

[Task  1] Index #20 started at 0ms and waited 875ms
[Task  2] Index #20 started at 0ms and waited 875ms
[Task  3] Index #20 started at 0ms and waited 875ms
[Task  4] Index #20 started at 0ms and waited 875ms
[Task  5] Index #20 started at 0ms and waited 875ms
[Task  6] Index #20 started at 0ms and waited 875ms
[Task  7] Index #20 started at 855ms and waited 1477ms
[Task  8] Index #20 started at 886ms and waited 1965ms
[Task  9] Index #20 started at 886ms and waited 1965ms

並行輸出

Task  6 started at 1046 ms and waited 636 ms
Task 11 started at 1561 ms and waited 758 ms
Task  0 started at 16 ms and waited 2891 ms
Task  5 started at 16 ms and waited 2891 ms
Task 15 started at 17 ms and waited 2891 ms
Task  1 started at 17 ms and waited 2891 ms
Task 10 started at 17 ms and waited 2891 ms

使用實際方法我也有相同的經驗，其中最后一項被多次返回而不是單獨的結果。

如果你能引導我朝着正確的方向前進，那將會有很大的幫助。

注意：輸出是部分的。 實際產量各有20項。

Answer 1

當使用Tasks方法時，你在'i'上有一個閉包，這就是為什么所有的任務ID都是20（即在任務實際執行時，所有20個都已經被安排，因此i == 20，當方法執行時）。

Answer 2

你需要在i上形成一個閉包。 這是最簡單的方法：

for (int x = 0; x < 20; x++)
{
    var i = x;
    tasks.Add(...) // keep using `i` inside here & never `x`
}

那應該解決它。

Answer 3

Parallel主要用於擴展算法以使用計算機上的可用CPU資源。 如果問題不是CPU綁定（即調用WebServices）並行不是正確的抽象。

所以在你的情況下，似乎任務是正確的選擇。