Question

我正试图围绕整个并行编程概念，主要关注任务，所以我一直在尝试这种情况，例如，最多9个并行任务将在一段随机的时间内完成他们的工作：

/// <remarks>
/// A big thank you to the awesome community of StackOverflow for 
/// their advice and guidance with this sample project
/// http://stackoverflow.com/questions/5195486/
/// </remarks>

RNGCryptoServiceProvider random = new RNGCryptoServiceProvider();
byte[] buffer = new byte[4];
random.GetBytes(buffer);

// Creates a random number of tasks (not less than 2 -- up to 9)
int iteration = new Random(BitConverter.ToInt32(buffer, 0)).Next(2,9);
Console.WriteLine("Creating " + iteration + " Parallel Tasks . . .");
Console.Write(Environment.NewLine);

Dictionary<int, string> items = new Dictionary<int, string>();

for (int i = 1; i < iteration + 1; i++) // cosmetic +1 to avoid "Task N° 0"
{
    items.Add(i, "Task # " + i);
}

List<Task> tasks = new List<Task>();

// I guess we should use a Parallel.Foreach() here
foreach (var item in items)
{
    // Creates a random interval to pause the thread at (up to 9 secs)
    random.GetBytes(buffer);
    int interval = new Random(BitConverter.ToInt32(buffer, 0)).Next(1000, 9000);

    var temp = item;
    var task = Task.Factory.StartNew(state =>
    {
        Console.WriteLine(String.Format(temp.Value + " will be completed in {0} miliseconds . . .", interval));
        Thread.Sleep(interval);

        return "The quick brown fox jumps over the lazy dog.";

    }, temp.Value).ContinueWith(t => Console.WriteLine(String.Format("{0} returned: {1}", t.AsyncState, t.Result)));

    tasks.Add(task);
}

Task.WaitAll(tasks.ToArray());

但不幸的是，它们是按顺序处理而不是并行处理。

如果你们能帮助我，我真的很高兴 - 也许我应该使用Parallel.ForEach而不是常规的？

再次提出任何建议都会非常感激。

修改更新了代码示例两次以反映评论者的贡献。

Answer 1

您正在循环中的每个任务上调用task.Result ...这意味着它将在创建下一个任务之前等待一个任务的结果。试试这个：

// Create all the tasks foreach (var item in items) { // ... Create and start tasks as before tasks.Add(task); } // Now wait for them all, printing the results foreach (var task in tasks) { Console.WriteLine(task.AsyncState + " returned: " + task.Result); }

现在这将阻止立即创建的第一个任务 - 所以即使（比方说）第5个任务更早完成，在第1个完成之前你不会看到任何结果......但它们将并行执行。

接下来你要改变你的循环：

foreach (var item in items) { var itemCopy = item; // Use itemCopy in here instead of item }

否则你正在捕获循环变量，这被认为是有害的（part 1; part 2）。

但是，您可能应该使用Parallel.ForEach。值得理解为什么它之前失败了。

Answer 2

调用Task.Result块。由于您在foreach项目中执行此操作，因此最终创建一项任务，然后等待其完成，然后再转到下一项目。

尝试将呼叫转移到Task.Result

之外的foreach

List<Task<string>> tasks = new List<Task<string>>();


foreach (var item in items)
{
    random.GetBytes(buffer);
    int interval = new Random(BitConverter.ToInt32(buffer, 0)).Next(1000, 9000);

    var task = Task.Factory.StartNew(state =>
    {
        Console.WriteLine(String.Format(item.Value + " will be completed in {0} miliseconds . . .", interval));
        Thread.Sleep(interval);

        return "The quick brown fox jumps over the lazy dog.";

    }, item.Value);

    tasks.Add(task);
}

foreach (var task in tasks)
{
    Console.WriteLine(task.AsyncState + " returned: " + task.Result);
}

修改

正如评论中所要求的那样。这是一个版本，可以在每个任务完成后使用ConinueWith打印结果。任务列表现在也可以再次为List<Task>。最后仍需要进行WaitAll调用，以确保方法在每个任务完成之前不会返回，但每个任务都会在完成后打印出结果。

RNGCryptoServiceProvider random = new RNGCryptoServiceProvider(); byte[] buffer = new byte[4]; random.GetBytes(buffer); // Creates a random number of tasks (not less than 2 -- up to 9) int iteration = new Random(BitConverter.ToInt32(buffer, 0)).Next(2, 9); Console.WriteLine("Creating " + iteration + " Parallel Tasks . . ."); Dictionary<int, string> items = new Dictionary<int, string>(); for (int i = 1; i < iteration + 1; i++) // cosmetic +1 to avoid "Task N° 0" { items.Add(i, "Parallel Task N° " + i); } List<Task> tasks = new List<Task>(); // I guess we should use a Parallel.Foreach() here foreach (var item in items) { // Creates a random interval to pause the thread at (up to 9 secs) random.GetBytes(buffer); int interval = new Random(BitConverter.ToInt32(buffer, 0)).Next(1000, 9000); // http://stackoverflow.com/questions/5195486/ var temp = item; var task = Task.Factory.StartNew(state => { Console.WriteLine(String.Format(temp.Value + " will be completed in {0} miliseconds . . .", interval)); Thread.Sleep(interval); return "The quick brown fox jumps over the lazy dog."; }, temp.Value).ContinueWith(t => Console.WriteLine(t.AsyncState + " returned: " + t.Result)); tasks.Add(task); } Task.WaitAll(tasks.ToArray());

Answer 3

由于您在每个任务上调用task.Result，因此您正在创建一个阻止调用，导致循环等待该任务返回。

您应首先创建所有任务，然后在单独的循环中读取结果。

并行任务按顺序处理

3 个答案: