Task.WhenAll
。我的任务经常失败,在这种情况下,我会用消息框通知用户,以便她可以重试。我的问题是报告错误被延迟,直到所有任务完成。相反,我想在第一个任务抛出异常时立即通知用户。换句话说,我想要一个快速失败的 Task.WhenAll
版本。由于不存在这样的内置方法,我尝试创建自己的方法,但我的实现不符合我想要的方式。这是我想到的:
public static async Task<TResult[]> WhenAllFailFast<TResult>(
params Task<TResult>[] tasks)
{
foreach (var task in tasks)
{
await task.ConfigureAwait(false);
}
return await Task.WhenAll(tasks).ConfigureAwait(false);
}
这通常比原生的
Task.WhenAll
抛出得更快,但通常不够快。在任务 #1 完成之前,不会观察到出现故障的任务 #2。我该如何改进它才能尽快失败?
更新:关于取消,目前不在我的要求之内,但可以说,为了保持一致性,第一个取消的任务应该立即停止等待。在这种情况下,从
WhenAllFailFast
返回的组合任务应该有 Status == TaskStatus.Canceled
。
澄清:取消场景是指用户单击“取消”按钮来停止任务完成。这并不是在出现异常时自动取消未完成的任务。
WhenAllFailFast
TaskCompletionSource
方法。您可以使用同步延续来 .ContinueWith() 每个输入任务,当任务以“Faulted”状态结束时(使用相同的异常对象),该延续会导致 TCS 出错。也许类似(未完全测试):
using System;
using System.Threading;
using System.Threading.Tasks;
namespace stackoverflow
{
class Program
{
static async Task Main(string[] args)
{
var cts = new CancellationTokenSource();
cts.Cancel();
var arr = await WhenAllFastFail(
Task.FromResult(42),
Task.Delay(2000).ContinueWith<int>(t => throw new Exception("ouch")),
Task.FromCanceled<int>(cts.Token));
Console.WriteLine("Hello World!");
}
public static Task<TResult[]> WhenAllFastFail<TResult>(params Task<TResult>[] tasks)
{
if (tasks is null || tasks.Length == 0) return Task.FromResult(Array.Empty<TResult>());
// defensive copy.
var defensive = tasks.Clone() as Task<TResult>[];
var tcs = new TaskCompletionSource<TResult[]>();
var remaining = defensive.Length;
Action<Task> check = t =>
{
switch (t.Status)
{
case TaskStatus.Faulted:
// we 'try' as some other task may beat us to the punch.
tcs.TrySetException(t.Exception.InnerException);
break;
case TaskStatus.Canceled:
// we 'try' as some other task may beat us to the punch.
tcs.TrySetCanceled();
break;
default:
// we can safely set here as no other task remains to run.
if (Interlocked.Decrement(ref remaining) == 0)
{
// get the results into an array.
var results = new TResult[defensive.Length];
for (var i = 0; i < tasks.Length; ++i) results[i] = defensive[i].Result;
tcs.SetResult(results);
}
break;
}
};
foreach (var task in defensive)
{
task.ContinueWith(check, default, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
}
return tcs.Task;
}
}
}
:解开AggregateException,取消支持,返回结果数组。防御数组突变、null 和empty。显式任务调度程序。
WhenAllFailFast
方法,并且我修改了@ZaldronGG的
优秀解决方案以使其性能更高一些(并且更符合Stephen Cleary的建议)。下面的实现在我的 PC 中每秒处理大约 3,500,000 个任务。
public static Task<TResult[]> WhenAllFailFast<TResult>(params Task<TResult>[] tasks)
{
if (tasks is null) throw new ArgumentNullException(nameof(tasks));
if (tasks.Length == 0) return Task.FromResult(new TResult[0]);
var results = new TResult[tasks.Length];
var remaining = tasks.Length;
var tcs = new TaskCompletionSource<TResult[]>(
TaskCreationOptions.RunContinuationsAsynchronously);
for (int i = 0; i < tasks.Length; i++)
{
var task = tasks[i];
if (task == null) throw new ArgumentException(
$"The {nameof(tasks)} argument included a null value.", nameof(tasks));
HandleCompletion(task, i);
}
return tcs.Task;
async void HandleCompletion(Task<TResult> task, int index)
{
try
{
var result = await task.ConfigureAwait(false);
results[index] = result;
if (Interlocked.Decrement(ref remaining) == 0)
{
tcs.TrySetResult(results);
}
}
catch (OperationCanceledException)
{
tcs.TrySetCanceled();
}
catch (Exception ex)
{
tcs.TrySetException(ex);
}
}
}
您可能会发现这篇文章对第一次失败后中止的模式很有帮助:
http://gigi.nullneuron.net/gigilabs/patterns-for-asynchronous-composite-tasks-in-c/
public static async Task<TResult[]> WhenAllFailFast<TResult>(
params Task<TResult>[] tasks)
{
var taskList = tasks.ToList();
while (taskList.Count > 0)
{
var task = await Task.WhenAny(taskList).ConfigureAwait(false);
if(task.Exception != null)
{
// Left as an exercise for the reader:
// properly unwrap the AggregateException;
// handle the exception(s);
// cancel the other running tasks.
throw task.Exception.InnerException;
}
taskList.Remove(task);
}
return await Task.WhenAll(tasks).ConfigureAwait(false);
}
async void
上启动多个
SynchronizationContext
操作有点怀疑。我在这里提出的解决方案要慢得多。它比 @ZaldronGG 的优秀解决方案慢了大约 3 倍,比我之前基于
async void
的实现慢了大约 10 倍。它的优点是,在完成返回的
Task<TResult[]>
后,它不会泄漏观察到的任务所附加的即发即忘的延续。当此任务完成时,所有由 WhenAllFailFast
方法内部创建的延续都已被清理。对于 API 来说,哪种行为是理想的行为是普遍的,但在许多情况下它可能并不重要。public static Task<TResult[]> WhenAllFailFast<TResult>(params Task<TResult>[] tasks)
{
ArgumentNullException.ThrowIfNull(tasks);
CancellationTokenSource cts = new();
Task<TResult> failedTask = null;
TaskContinuationOptions flags = TaskContinuationOptions.DenyChildAttach |
TaskContinuationOptions.ExecuteSynchronously;
Action<Task<TResult>> continuationAction = new(task =>
{
if (!task.IsCompletedSuccessfully)
if (Interlocked.CompareExchange(ref failedTask, task, null) is null)
cts.Cancel();
});
IEnumerable<Task> continuations = tasks.Select(task => task
.ContinueWith(continuationAction, cts.Token, flags, TaskScheduler.Default));
return Task.WhenAll(continuations).ContinueWith(allContinuations =>
{
cts.Dispose();
Task<TResult> localFailedTask = Volatile.Read(ref failedTask);
if (localFailedTask is not null)
return Task.WhenAll(localFailedTask);
// At this point all the tasks are completed successfully
Debug.Assert(tasks.All(t => t.IsCompletedSuccessfully));
Debug.Assert(allContinuations.IsCompletedSuccessfully);
return Task.WhenAll(tasks);
}, default, flags, TaskScheduler.Default).Unwrap();
}
此实现与 ZaldronGG 的类似,它为每个任务附加一个延续,不同之处在于这些延续是可以取消的,并且当观察到第一个不成功的任务时,它们会被集体取消。它还使用了我最近发现的
Unwrap
技术
,这消除了手动完成
TaskCompletionSource<TResult[]>
实例的需要,并且通常可以实现简洁的实现。