在 HttpClient 中使用 await 的异步调用永远不会返回 [英] Async call with await in HttpClient never returns

问题描述

我在 Win8 CP 上的基于 xaml 的 C# 地铁应用程序中进行了调用；这个调用只是访问一个网络服务并返回 JSON 数据.

I have a call I am making from inside a xaml-based, C# metro application on the Win8 CP; this call simply hits a web service and returns JSON data.

HttpMessageHandler handler = new HttpClientHandler();

HttpClient httpClient = new HttpClient(handler);
httpClient.BaseAddress = new Uri("http://192.168.1.101/api/");

var result = await httpClient.GetStreamAsync("weeklyplan");
DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(WeeklyPlanData[]));
return (WeeklyPlanData[])ser.ReadObject(result);

它挂在 await 处，但 http 调用实际上几乎立即返回(通过 fiddler 确认)；就好像 await 被忽略了，它只是挂在那里.

It hangs at the await but the http call actually returns almost immediately (confirmed through fiddler); it is as if the await is ignored and it just hangs there.

在您询问之前 - 是的 - 专用网络功能已打开.

Before you ask - YES - the Private Network capability is turned on.

任何想法为什么会挂起?

Any ideas why this would hang?

推荐答案

查看这个答案我的问题，这似乎非常相似.

Check out this answer to my question which seems to be very similar.

尝试一下:对 GetStreamAsync() 返回的任务调用 ConfigureAwait(false).例如

Something to try: call ConfigureAwait(false) on the Task returned by GetStreamAsync(). E.g.

var result = await httpClient.GetStreamAsync("weeklyplan")
                             .ConfigureAwait(continueOnCapturedContext:false);

这是否有用取决于你上面的代码是如何被调用的 - 在我的例子中使用 Task.GetAwaiter().GetResult() 调用 async 方法导致代码挂起.

Whether or not this is useful depends on how your code above is being called - in my case calling the async method using Task.GetAwaiter().GetResult() caused the code to hang.

这是因为 GetResult() 阻塞当前线程，直到任务完成.当任务完成时，它会尝试重新进入启动它的线程上下文，但无法重新进入，因为该上下文中已经有一个线程，该线程被调用 GetResult()... 僵局！

This is because GetResult() blocks the current thread until the Task completes. When the task does complete it attempts to re-enter the thread context in which it was started but cannot because there is already a thread in that context, which is blocked by the call to GetResult()... deadlock!

这篇 MSDN 帖子详细介绍了如何使用 .NET 同步并行线程 - 并且对我自己问题的回答提供了一些最佳实践.

This MSDN post goes into a bit of detail on how .NET synchronizes parallel threads - and the answer given to my own question gives some best practices.

这篇关于在 HttpClient 中使用 await 的异步调用永远不会返回的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！