TaskCanceledException时不等待 [英] TaskCanceledException when not awaiting

问题描述

我似乎得到一个 TaskCanceledException 每当我返回另一个任务，而不是同步等待它，在之后的最后当你等待。

I seem to be getting a TaskCanceledException whenever I return another Task synchronously instead of awaiting it, following the guidelines in When at last you await.

public static class Download
{
    public static Task<byte[]> FromYouTubeAsync(string videoUri)
    {
        using (var client = new HttpClient())
        {
            return FromYouTubeAsync(
                () => client
                .GetStringAsync(videoUri),
                uri => client
                .GetByteArrayAsync(uri));
        }
    }

    public async static Task<byte[]> FromYouTubeAsync(
        Func<Task<string>> sourceFactory, Func<string, Task<byte[]>> downloadFactory)
    {
        string source = await // TaskCanceledException here
            sourceFactory()
            .ConfigureAwait(false);

        // find links

        return await
            downloadFactory(links.First())
            .ConfigureAwait(false);
    }
}

异常免费code

下面，该方法的签名的第一过载被改变为异步，并且它等待第二过载。出于某种原因，这prevents的 TaskCanceledException 。

public static class Download
{
    public async static Task<byte[]> FromYouTubeAsync(string videoUri)
    {
        using (var client = new HttpClient())
        {
            return await FromYouTubeAsync(
                () => client
                .GetStringAsync(videoUri),
                uri => client
                .GetByteArrayAsync(uri));
        }
    }

    public async static Task<byte[]> FromYouTubeAsync(
        Func<Task<string>> sourceFactory, Func<string, Task<byte[]>> downloadFactory)
    {
        string source = await // No exception!
            sourceFactory()
            .ConfigureAwait(false);

        // find links

        return await
            downloadFactory(links.First())
            .ConfigureAwait(false);
    }
}

为什么会这样，我能做些什么来解决这个问题（除了等待方法，浪费了资源在上面的链接说明）？

Why does this happen and what can I do to fix it (besides awaiting the method, which wastes resources as described in the link above)?

推荐答案

不好意思，的链接你贴是关于应用优化的是仅适用如果该方法后，没有它的等待 的。引用帖子：

Sorry, the link you posted is about applying an optimization which is only applicable if the method does nothing after its await. To quote the post:

在此情况下，然而，我们被交给一个任务重新present在方法的最后一条语句，因此它实际上已经是一个再$整个方法的处理的对$ psentation ...

In this case, however, we’re being handed a task to represent the last statement in the method, and thus it’s in effect already a representation of the entire method’s processing...

在你的榜样，任务做的不的再present在方法的最后一条语句。再看一下：

In your example, the task does not represent the last statement in the method. Look again:

public async static Task<byte[]> FromYouTubeAsync(string videoUri)
{
  using (var client = new HttpClient())
  {
    return await FromYouTubeAsync(...);
  }
}

有后出事的 的的等待：具体而言，客户端的处置。所以在这个博客中提到的最优化的在这里并不适用的

There's something happening after the await: specifically, the disposing of client. So the optimization mentioned in that blog post does not apply here.

这就是为什么你看到一个异常，如果您尝试直接返回任务：

This is why you're seeing an exception if you try to return the task directly:

public static Task<byte[]> FromYouTubeAsync(string videoUri)
{
  using (var client = new HttpClient())
  {
    return FromYouTubeAsync(...);
  }
}

这code开始下载，则配置了的HttpClient 的，然后返回任务。 的HttpClient 它处置时，将取消所有未完成的操作。

This code is starting the download, then disposing the HttpClient, and then returning the task. HttpClient will cancel any outstanding operations when it is disposed.

使用code 等待将（异步）等待HTTP操作来完成其配置在<​​code>的HttpClient 。这是你需要的行为，而等待是前preSS是最彻底的方法。在这种情况下，它不是一个资源浪费可言，因为你的有无的推迟，直到处理完成下载后。

The code using await will (asynchronously) wait for the HTTP operation to complete before it disposes the HttpClient. That is the behavior you need, and await is the cleanest way to express it. In this case, it's not a "waste of resources" at all, because you have to defer disposing until after the download completes.

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