HttpClient等待SendAsync死锁

Question

我已经和这件事斗争了好几天了，我不知道为什么它是这样的。我希望你们中的一个能给我解释一下。

以下是有关的守则：

public async static Task<RemoteFileDetails> GetRemoteFileDetailsAsync(string strUrl, double dblTimeoutSeconds = 15.0)
{
    try
    {
        using (var hc = new HttpClient())
        {
            hc.Timeout = TimeSpan.FromSeconds(dblTimeoutSeconds);
            using (var h = new HttpRequestMessage(HttpMethod.Head, strUrl))
            {
                h.Headers.UserAgent.ParseAdd(UserAgent);
                using (var rm = await hc.SendAsync(h))
                {
                    rm.EnsureSuccessStatusCode();
                    return new RemoteFileDetails()
                    {
                        Url = strUrl,
                        FileName = rm.Content.Headers.ContentDisposition.FileName,
                        FileSize = rm.Content.Headers.ContentLength.GetValueOrDefault(),
                        LastModified = rm.Content.Headers.LastModified.GetValueOrDefault().LocalDateTime,
                        Valid = true
                    };
                }
            }
        }
    }
    catch (Exception ex)
    {
        System.Windows.MessageBox.Show(ex.ToString());
    }
    return new RemoteFileDetails();
}

所做的就是在URL上输入一个HEAD请求并获取特定的标头结果。

这里的问题是大约50%的时间它死锁(暂停60秒左右，然后抛出一个TaskCanceledException)，其余的50%，它的工作出色。现在，我添加了网络日志记录，这就是死锁发生的情况：

    System.Net Verbose: 0 : [2928] HttpWebRequest#3567399::HttpWebRequest(https://www.example.com/file.txt#-114720138)
    System.Net Information: 0 : [2928] Current OS installation type is 'Client'.
    System.Net Information: 0 : [2928] RAS supported: True
    System.Net Verbose: 0 : [2928] Exiting HttpWebRequest#3567399::HttpWebRequest() 
    System.Net Verbose: 0 : [2928] HttpWebRequest#3567399::HttpWebRequest(uri: 'https://www.example.com/file.txt', connectionGroupName: '565144')
    System.Net Verbose: 0 : [2928] Exiting HttpWebRequest#3567399::HttpWebRequest() 
    System.Net Verbose: 0 : [5456] HttpWebRequest#3567399::BeginGetResponse()
    System.Net Verbose: 0 : [2244] HttpWebRequest#3567399::Abort()
    System.Net Error: 0 : [2244] Exception in HttpWebRequest#3567399:: - The request was aborted: The request was canceled..
/////// THIS IS WHERE THE 30-60 SECOND PAUSE HAPPENS ////////
    System.Net Error: 0 : [5456] Can't retrieve proxy settings for Uri 'https://www.example.com/file.txt'. Error code: 12180.
    System.Net Verbose: 0 : [5456] ServicePoint#39086322::ServicePoint(www.example.com:443)
    System.Net Information: 0 : [5456] Associating HttpWebRequest#3567399 with ServicePoint#39086322
    System.Net Verbose: 0 : [2244] Exiting HttpWebRequest#3567399::Abort() 
    System.Net Verbose: 0 : [5456] HttpWebRequest#3567399::EndGetResponse()
    System.Net Error: 0 : [5456] Exception in HttpWebRequest#3567399::EndGetResponse - The request was aborted: The request was canceled..
    System.Net Verbose: 0 : [5456] Exiting HttpWebRequest#3567399::BeginGetResponse()   -> ContextAwareResult#36181605

那么，看起来HttpClient()对象正在被释放吗？但是怎么做呢？为了验证是否存在这种情况，我将代码更改为：

private static HttpClient _hc;
public async static Task<RemoteFileDetails> GetRemoteFileDetailsAsync(string strUrl, double dblTimeoutSeconds = 15.0)
{
    _hc = new HttpClient();
    try
    {
        _hc.Timeout = TimeSpan.FromSeconds(dblTimeoutSeconds);
        using (var h = new HttpRequestMessage(HttpMethod.Head, strUrl))
        {
            h.Headers.UserAgent.ParseAdd(UserAgent);
            using (var rm = await _hc.SendAsync(h))
            {
                rm.EnsureSuccessStatusCode();
                return new RemoteFileDetails()
                {
                    Url = strUrl,
                    FileName = rm.Content.Headers.ContentDisposition.FileName,
                    FileSize = rm.Content.Headers.ContentLength.GetValueOrDefault(),
                    LastModified = rm.Content.Headers.LastModified.GetValueOrDefault().LocalDateTime,
                    Valid = true
                };
            }
        }
    }
    catch (Exception ex)
    {
        System.Windows.MessageBox.Show(ex.ToString());
    }
    return new RemoteFileDetails();
}

但它还是死死的。

为什么会发生这种事，有什么解释吗？我认为异步/等待基本上是“暂停”例程，所以无论如何，HttpClient都不应该释放。

还是我完全错过了这条船，这是另一回事。

这里是一个例外的尖叫声：

​

ETA:，我想展示一下我是如何称呼它的是很重要的。这是一个使用的WPF应用程序。在主视图模型中，我这样称呼它：

    public MainViewModel(IDataService ds)
    {
        _dataService = ds;
        this.Initialize();
    }

    private async void Initialize()
    {
        var det = await RemoteFileUtils.GetRemoteFileDetailsAsync("https://example.com/file.txt");
}

Answer 1

这个问题源于它是如何被调用的。

不要在调用类的构造函数中调用它。也要避免async void，除非它在事件处理程序中。

public class MainViewModel : ViewModelBase {

    public MainViewModel(IDataService ds) {
        _dataService = ds;
        this.Initialize += OnInitialize;
        //Un-comment if you want to call event immediately
        //Initialize(this, EventArgs.Empty);
    }

    public event EventHandler Initialize;

    private async void OnInitialize(object sender, EventArgs e) {
        var det = await RemoteFileUtils.GetRemoteFileDetailsAsync("https://example.com/file.txt");
    }

    //Or call this after class has created.
    public void Ready() {
        Initialize(this, EventArgs.Empty);
    }
}

接下来，不要在每次需要发出请求时创建和解析一个新的HttpClient。创建一个实例并在整个应用程序的生命周期中使用该实例。

for each distinct API that you connect to.

GetRemoteFileDetailsAsync也应该重构为一个可注入的服务，而不是静态访问。