编写Task.WhenAll / WhenAny变体，以取消第一个故障/已取消任务上的所有其他任务 [英] Writing a Task.WhenAll/WhenAny variant that cancels all other tasks on first faulted/Cancelled task

问题描述

我是C＃的新手，今天开始使用TPL。我决定编写
Task Task.WhenAll 的修改版作为练习。我希望它具有以下行为：

I'm fairly new to C# and started playing around with the TPL today. I decided to write a modified version of Task Task.WhenAll as an exercise. I'd like for it to have the following behavior:

• 找到第一个已出错或被取消的任务后，请取消其余的


• 如果任务出错，则返回的任务应设置正确的异常（即不被继续执行并替换为OperationCancelledException（）吞噬） li>
  
• 方法签名中没有异步（希望避免冒泡）。

• Upon finding the first task that has faulted or been canceled, cancel the rest of the tasks instead of waiting for them to finish.
• If the task faulted, the returned task should have the right exception set (i.e no swallowing by continuation and replacing with OperationCancelledException())
• No async in the method signature (want to avoid bubbling it up).

我来了下面的疯狂/愚蠢的代码行不通，我很难想象发生了什么。我无法想象会发生什么阻碍，而我预想的是，一连串的任务都在等待其余的任务完成。

I came up with the following crazy/stupid piece of code that doesn't work and I am having a hard time visualizing what's going on. I can't imagine any blocking going on and what i envisioned happening was a chain of tasks each waiting on the rest for completion. Could someone explain what's going on?

我不会在生产代码中使用它，这只是为了测试我的基本知识。我意识到一种更简单的方法是执行Task.WhenAll并让列表中的任务本身具有连续性，这些连续性会在失败时进行取消。

I wouldn't have it in production code and this is just to test my fundamentals. I realize an easier way of doing this would be to do a Task.WhenAll and have the tasks in the list themselves have continuations that do the cancellation on failure.

    public static Task WhenAllError(List<Task> tasks, CancellationToken ct)
    {
        var tcs = new TaskCompletionSource<object>();
        return Task.WhenAny(tasks).ContinueWith<Task>((t) =>
             {
                 if (tasks.Count == 0)
                 {
                     tcs.SetResult(null);
                     return tcs.Task;
                 }

                 if (t.IsFaulted)
                 {
                     Console.WriteLine("Task faulted. Cancelling other tasks: {0}", t.Id);
                     cts.Cancel();
                     // Make sure the tasks are cancelled if necessary
                     tcs.SetException(t.Exception);
                     return tcs.Task;
                 }
                 // Similarly handle Cancelled

                 tasks.Remove(t);
                 return WhenAllError(tasks, ct);
             }).Unwrap();
    }

推荐答案

CancellationToken 类没有 Cancel 方法。您需要一个 CancellationTokenSource 即可取消 CancellationToken 。

The CancellationToken class has no Cancel method. You need a CancellationTokenSource to be able to cancel the CancellationToken.

同样，要影响任务的结果，您需要 TaskCompletionSource ，您无法安全地
取消已运行的任务。看到此帖子

Similarly to affect the outcome of a task you need a TaskCompletionSource you can't safely cancel already running tasks. See this post

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