如何等待直到Task.ContinueWith内部任务完成 [英] How to await until Task.ContinueWith inner task finishes

问题描述

看看下面的代码：

    private async Task InnerTask(bool outerTaskResult)
    {
        Console.WriteLine("2");
        await Task.Factory.StartNew(() => Thread.Sleep(10000));
        Console.WriteLine("3");
    }

    private async void Button2_Click(object sender, RoutedEventArgs e)
    {
        var task = Task.FromResult(false);
        Task<Task> aggregatedTask = task.ContinueWith(task1 => InnerTask(task1.Result));
        Console.WriteLine("1");
        await aggregatedTask;
        Console.WriteLine("4");
    }

所需的输出为：

1
2
3
4

但是我得到：

1
2
4
3

这可能与在不同线程上执行InnerTask有关。

This probably has something to do with InnerTask being executed on a different thread.

我使用ContinueWith是因为原始代码中的任务是通过这种方式动态创建和排队的。

I'm using ContinueWith because the tasks in the original code are dynamically created and queued this way.

使用 .Wait（）方法（见下文）有效，但我认为这是一个坏主意，因为该方法正在阻塞。

Using .Wait() method (see below) works, but I think it's a bad idea, as the method is blocking.

task.ContinueWith(task1 => InnerTask(task1.Result).Wait())

这里的正确方法是什么？

What's the correct approach here?

推荐答案

您可以使用 TaskExtensions.Unwrap（）（这是 Task< Task> 的扩展方法）来展开外部任务并检索内部任务：

You can use TaskExtensions.Unwrap() (which is an extension method on Task<Task>) to unwrap the outter task and retrieve the inner one:

private async void Button2_Click(object sender, RoutedEventArgs e)
{
    var task = Task.FromResult(false);
    Task aggregatedTask = task.ContinueWith(task1 => InnerTask(task1.Result)).Unwrap();
    Console.WriteLine("1");
    await aggregatedTask;
    Console.WriteLine("4");
}

请注意，为了简化整件事，而不是 ContinueWith 样式延续，您可以等待完成任务：

Note that to simplify this entire thing, instead of ContinueWith style continuation you can await on your tasks:

private async void Button2_Click(object sender, RoutedEventArgs e)
{
    var task = Task.FromResult(false);

    Console.WriteLine("1");

    var result = await task;
    await InnerTask(result);

    Console.WriteLine("4");
}

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