如何正确使用Task.WhenAll（） [英] How to use Task.WhenAll() correctly

问题描述

我想使用Task.WhenAll等待多个任务的完成。

I am trying to use Task.WhenAll to await completion of multiple tasks.

我code是低于 - 它应该启动多个异步任务，每个检索公共汽车路线，然后将它们添加到一个本地阵列。然而，Task.WhenAll（...）立即返回，并且本地路由阵列的计数是零。这似乎很奇怪，因为我期望的各种'等待'每个任务中的语句，意味着流被暂停，并且任务不返回，直到它完成。

My code is below - it is supposed to launch multiple async tasks, each of which retrieves a bus route and then adds them to a local array. However, Task.WhenAll(...) returns immediately, and the count of the local routes array is zero. This seems strange, since I would expect the various 'await' statements within each Task to mean that the flow is suspended, and the Task does not return until it's finished.

        List<Task> monitoredTasks = new List<Task>();
        foreach (BusRouteIdentifier bri in stop.services)
        {
            BusRouteRequest req = new BusRouteRequest(bri.id);

            // Start a new task to fetch the route for each stop
            Task getRouteTask = Task.Factory.StartNew(async () =>
            {
                var route = await BusDataProviderManager.DataProvider.DataBroker.getRoute(req);

                    // Add the route to our array (on UI thread as it's observed)
                    await dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, delegate
                    {
                        this.routes.Add(route);
                    });
            });

            // Store the task in our monitoring list
            monitoredTasks .Add(getRouteTask);
        }

        Debug.WriteLine("Awaiting WHENALL");
        await Task.WhenAll(monitoredTasks );
        Debug.WriteLine(string.Format("WHENALL returned (routes count is {0} ", this.routes.Count));

        this.OnWillEndFetchingRoutes(new EventArgs());

我明明做错了 - 但什么

I'm obviously doing something wrong - but what?

推荐答案

这是下降到一个基本的缺乏如何异步伺机真正起作用的理解。

This was down to a basic lack of understanding of how async-await really works.

内任务返回流动到外任务时，伺机以往任何时候都返回之前，然后结束了。

The inner task was returning flow to the outer task, which then finished before the await ever returned.

要实现我想要的东西，我需要重构如下：

To achieve what I wanted, I needed to refactor as follows:

        List<Task<BusRoute>> routeRetrievalTasks = new List<Task<BusRoute>>();
        foreach (BusRouteIdentifier bri in stop.services)
        {
            BusRouteRequest req = new BusRouteRequest(bri.id);
            routeRetrievalTasks.Add(BusDataProviderManager.DataProvider.DataBroker.getRoute(req));
        }

        foreach (var task in routeRetrievalTasks)
        {
            var route = await task;
            this.routes.Add(route); // triggers events
        }

感谢<一href="http://social.msdn.microsoft.com/Forums/en-US/winappswithcsharp/thread/51114ea5-0070-4285-b16e-231a895a6ad7">Dave史密茨

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