我需要在控制台应用程序中运行多个异步任务，并在进一步处理之前等待它们全部完成。
有很多文章，但我似乎越读越困惑。我已经阅读并理解了Task库的基本原理，但我显然在某个地方遗漏了一个链接。
我知道可以将任务串联起来，这样它们就可以在另一个任务完成后开始(这几乎是我读过的所有文章的场景)，但我希望所有任务同时运行，并且我想知道它们都完成后的情况。
对于这样的场景，最简单的实现是什么?
当前回答
我所见过的最佳选择是以下扩展方法:
public static Task ForEachAsync<T>(this IEnumerable<T> sequence, Func<T, Task> action) {
return Task.WhenAll(sequence.Select(action));
}
这样叫它:
await sequence.ForEachAsync(item => item.SomethingAsync(blah));
或者使用async lambda:
await sequence.ForEachAsync(async item => {
var more = await GetMoreAsync(item);
await more.FrobbleAsync();
});
其他回答
您希望将任务链接起来，还是可以以并行的方式调用它们?
的链接 就像这样
Task.Run(...).ContinueWith(...).ContinueWith(...).ContinueWith(...);
Task.Factory.StartNew(...).ContinueWith(...).ContinueWith(...).ContinueWith(...);
不要忘记检查每个ContinueWith中的前一个Task实例，因为它可能有错误。
对于平行方式 我遇到的最简单的方法是:平行。调用 除此之外还有任务。WaitAll或者你甚至可以使用WaitHandles来做一个倒数到零的动作(等待，有一个新的类:CountdownEvent)，或者…
我所见过的最佳选择是以下扩展方法:
public static Task ForEachAsync<T>(this IEnumerable<T> sequence, Func<T, Task> action) {
return Task.WhenAll(sequence.Select(action));
}
这样叫它:
await sequence.ForEachAsync(item => item.SomethingAsync(blah));
或者使用async lambda:
await sequence.ForEachAsync(async item => {
var more = await GetMoreAsync(item);
await more.FrobbleAsync();
});
你可以使用WhenAll，它将返回一个可等待的任务，或者WaitAll，它没有返回类型，将阻止进一步的代码执行，类似于线程。休眠，直到所有任务都完成、取消或出错。
WhenAll |
WaitAll |
|
|---|---|---|
| Any of the supplied tasks completes in a faulted state | A task with the faulted state will be returned. The exceptions will contain the aggregation of the set of unwrapped exceptions from each of the supplied tasks. | An AggregateException will be thrown. |
| None of the supplied tasks faulted but at least one of them was canceled | The returned task will end in the TaskStatus.Canceled state |
An AggregateException will be thrown which contains an OperationCanceledException in its InnerExceptions collection |
| An empty list was given | An ArgumentException will be thrown |
The returned task will immediately transition to a TaskStatus.RanToCompletion State before it's returned to the caller. |
| Doesn't block the current thread | Blocks the current thread |
例子
var tasks = new Task[] {
TaskOperationOne(),
TaskOperationTwo()
};
Task.WaitAll(tasks);
// or
await Task.WhenAll(tasks);
如果你想以特定的顺序运行任务，你可以从这个答案中得到灵感。
如果你正在使用async/await模式，你可以像这样并行运行几个任务:
public async Task DoSeveralThings()
{
// Start all the tasks
Task first = DoFirstThingAsync();
Task second = DoSecondThingAsync();
// Then wait for them to complete
var firstResult = await first;
var secondResult = await second;
}
我准备了一段代码，向您展示如何在这些场景中使用该任务。
// method to run tasks in a parallel
public async Task RunMultipleTaskParallel(Task[] tasks) {
await Task.WhenAll(tasks);
}
// methode to run task one by one
public async Task RunMultipleTaskOneByOne(Task[] tasks)
{
for (int i = 0; i < tasks.Length - 1; i++)
await tasks[i];
}
// method to run i task in parallel
public async Task RunMultipleTaskParallel(Task[] tasks, int i)
{
var countTask = tasks.Length;
var remainTasks = 0;
do
{
int toTake = (countTask < i) ? countTask : i;
var limitedTasks = tasks.Skip(remainTasks)
.Take(toTake);
remainTasks += toTake;
await RunMultipleTaskParallel(limitedTasks.ToArray());
} while (remainTasks < countTask);
}
