（问题的真正标题应该是为什么我得到无法投类型的对象System.Runtime.CompilerServices.TaskAwaiter`1 [System.Runtime.CompilerServices.VoidTaskResult]'输入'系统。 Runtime.CompilerServices.INotifyCompletion'，但不幸的是，这是太长了计算器：）

(The real title of the question should be "Why do I get a 'Unable to cast object of type 'System.Runtime.CompilerServices.TaskAwaiter`1[System.Runtime.CompilerServices.VoidTaskResult]' to type 'System.Runtime.CompilerServices.INotifyCompletion'", but unfortunately this is too long for StackOverflow. :)

您好，

我试图时变得非常奇特的问题接踵而至我的一个方法的执行。调用code如下（节选）：

I'm getting really peculiar problems when trying to await the execution of a method of mine. The calling code looks like this (excerpt):

    private async Task DownloadAddonFileAsync(dynamic addon, dynamic file, string targetFolder)
    {
        // ...
        await DownloadFileAsync(file, targetFolder, uri);

在DownloadFileAsync看起来是这样的：

The DownloadFileAsync looks like this:

    protected async Task DownloadFileAsync(dynamic file, string targetFolder, string uri)
    {
        // ...
        var fileBytes = await AppLoaderRestClient.GetAsync<byte[]>(uri);

在AppLoaderRestClient.GetAsync（），反过来，看起来是这样的：

The AppLoaderRestClient.GetAsync(), in turn, looks like this:

    public static async Task<T> GetAsync<T>(string uri)
    {
        // ...
        if (typeof (T) == typeof (byte[]))
        {
            var result = await webClient.DownloadDataTaskAsync(uri);
            return (T) (object) result;
        }

所以，其实是有链的任务在这里 - 内部任务将是一个任务，然后将被传播到调用者，并将其转换为一个任务（即任务无的结果）。我presume这可能手头会造成这个问题？

So, there is actually a chain of Tasks here - the "inner" Task will be a Task, which will then be propagated up to the caller, and converted to a Task (i.e. a Task without a result). I presume this could be causing the issue at hand?

如果我更改最外面的code到这一点：

If I change the outermost code to this:

        var task = DownloadFileAsync(file, targetFolder, uri);
        task.Wait();

...它完美的作品。为什么呢？

...it works flawlessly. Why?

推荐答案

由于它有时会证明，我设法找到了问题的答案，而在这里张贴。想我会分享它救别人头痛...

As it sometimes turns out, I managed to find out the answer to the question while posting it here. Thought I'd share it to save someone else from headache...

这是我使用的动态，或者更确切地说，问题的根源时，（在我看来）轻度受限和破碎的方式动态作品在C＃/。NET的呢。如果我重新整理我的code是这样的：

The problem stems from my use of dynamic, or rather, the (in my opinion) slightly limited and broken way dynamic works in C#/.NET as of yet. If I rephrase my code like this:

await (Task)DownloadFileAsync(file, targetFolder, uri);

...它完美的作品。

...it works flawlessly.

下面的事情是，因为我的参数之一（文件是动态的），这将是一个动态操作。和返回值似乎被某种方式从动态操作搞砸了; CLR的是根本无法从code来推断上述是否该方法返回工作或任务＆LT; T＆GT; （左右我猜）。因此，它失败尝试的结果强制转换为INotifyCompletion实例 - 因此，除了

The thing here is that since one of my parameters (file is dynamic), this will be a dynamic operation. And return values seem to be somehow "messed up" from dynamic operations; the CLR is simply unable to deduce from the code above whether the method returns Task or Task<T> (or so I guess). It therefore fails trying to cast the result to an INotifyCompletion instance - hence, the exception.

感谢的很多的微软。 ;） （我觉得这里的主要问题是，异常消息是很清楚，在我看来...）

Thanks a lot, Microsoft. ;) (I think the main problem here is that the exception message was very unclear, in my opinion...)