Question

这是几周来一直运行良好的原始代码。在我刚刚做的测试中，它在100次尝试中失败了。

using (var httpClient = new HttpClient())
{
    var tasks = new List<Task>();

    tasks.Add(httpClient.GetAsync(new Uri("..."))
        .ContinueWith(request =>
        {
            request.Result.Content.ReadAsAsync<IEnumerable<Foo>>()
                .ContinueWith(response =>
                {
                    foos = response.Result;
                });
        }));

    tasks.Add(httpClient.GetAsync(new Uri("..."))
        .ContinueWith(request =>
        {
            request.Result.Content.ReadAsAsync<Bar>()
                .ContinueWith(response =>
                {
                    bar = response.Result;
                });
        }));

    await Task.WhenAll(tasks);
}

此代码在100次尝试中失败了9次，其中一个或两个元组值为null。

var APIresponses = await HttpClientHelper.GetAsync
    <
        IEnumerable<Foo>,
        Bar
    >
    (
        new Uri("..."),
        new Uri("...")
    );

foos = APIresponses.Item1;
bar = APIresponses.Item2;

private static Task GetAsync<T>(HttpClient httpClient, Uri URI, Action<Task<T>> continuationAction)
{
    return httpClient.GetAsync(URI)
        .ContinueWith(request =>
        {
            request.Result.EnsureSuccessStatusCode();

            request.Result.Content.ReadAsAsync<T>()
                .ContinueWith(continuationAction);
        });
}

public static async Task<Tuple<T1, T2>> GetAsync<T1, T2>(Uri URI1, Uri URI2)
{
    T1 item1 = default(T1);
    T2 item2 = default(T2);

    var httpClient = new HttpClient();
    var tasks = new List<Task>()
    {
        GetAsync<T1>(httpClient, URI1, response =>
        {
            item1 = response.Result;
        }),
        GetAsync<T2>(httpClient, URI2, response =>
        {
            item2 = response.Result;
        })
    };

    await Task.WhenAll(tasks);

    return Tuple.Create(item1, item2);
}

将代码修改为如此，并且在100次尝试中它将再次失败。

    await Task.WhenAll(tasks);
    System.Diagnostics.Debug.WriteLine("tasks complete");
    System.Diagnostics.Debug.WriteLine(item1);
    System.Diagnostics.Debug.WriteLine(item2);

    return Tuple.Create(item1, item2);
}

我已经看了半个多小时，但我没看到错误在哪里。有人看到了吗？

Answer 1

此代码：

        request.Result.Content.ReadAsAsync<T>()
            .ContinueWith(continuationAction);

返回一个任务，但永远不会等待该任务（并且不会向其添加Continuation）。因此，在Task.WhenAll返回之前，可能无法设置项目。

然而，原始解决方案似乎也有同样的问题。

我的猜测是你正在处理值类型，并且两者都有竞争条件，但在第二个例子中，你很早就将值类型（虽然它们仍然是它们的默认值）复制到元组中。在其他示例中，您需要等待很长时间才能复制或使用它们，以便设置值的问题延续运行。

Answer 2

要解决来自other question的评论，您很少需要将async / await与ContinueWith混在一起。您可以在async lambdas的帮助下执行“fork”逻辑，例如，问题中的代码可能如下所示：

using (var httpClient = new HttpClient())
{
    Func<Task<IEnumerable<Foo>>> doTask1Async = async () =>
    {
        var request = await httpClient.GetAsync(new Uri("..."));
        return response.Content.ReadAsAsync<IEnumerable<Foo>>();
    };

    Func<Task<IEnumerable<Bar>>> doTask2Async = async () =>
    {
        var request = await httpClient.GetAsync(new Uri("..."));
        return response.Content.ReadAsAsync<IEnumerable<Bar>>();
    };

    var task1 = doTask1Async();
    var task2 = doTask2Async();

    await Task.WhenAll(task1, task2);

    var result1 = task1.Result;
    var result2 = task2.Result;

    // ...
}

Answer 3

编辑：不接受我自己的答案，但留待参考。代码工作，带有catch：ContinueWith loses the SynchronizationContext

感谢@jbl和@MattSmith让我走上正轨。

问题确实是Task.WhenAll不等待继续。解决方案是设置TaskContinuationOptions.AttachedToParent。

所以这个

private static Task GetAsync<T>(HttpClient httpClient, Uri URI, Action<Task<T>> continuationAction)
{
    return httpClient.GetAsync(URI)
        .ContinueWith(request =>
        {
            request.Result.EnsureSuccessStatusCode();

            request.Result.Content.ReadAsAsync<T>()
                .ContinueWith(continuationAction);
        });
}

变成这个

private static Task GetAsync<T>(HttpClient httpClient, Uri URI, Action<Task<T>> continuationAction)
{
    return httpClient.GetAsync(URI)
        .ContinueWith(request =>
        {
            request.Result.EnsureSuccessStatusCode();

            request.Result.Content.ReadAsAsync<T>()
                .ContinueWith(continuationAction, TaskContinuationOptions.AttachedToParent);
        }, TaskContinuationOptions.AttachedToParent);
}

有关MSDN: Nested Tasks and Child Tasks

的更多信息

为什么这个异步代码有时会失败，只有在没有观察到的情况下？

3 个答案: