使用语句中的HttpClient导致任务被取消

Question

我FileResult : IHttpActionResult为api调用创建了一个webapi返回类型.FileResult从另一个URL下载文件,然后将流返回给客户端.

最初我的代码有using如下声明:

public async Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
{
    try
    {
        HttpResponseMessage response;
        using (var httpClient = new HttpClient())
        {

            response = new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = new System.Net.Http.StreamContent(
                                    await httpClient.GetStreamAsync(this.filePath))
            };
        }
        return response;
    }
    catch (WebException exception)
    {...}
}

然而,这会间歇性地导致TaskCanceledException.我知道如果在异步调用完成之前处理了HttpClient,Task的状态将变为取消.但是因为我使用了await:Content = new System.Net.Http.StreamContent(await httpClient.GetStreamAsync(this.filePath))这应该可以防止HttpClient在任务完成过程中被丢弃.

为什么该任务被取消？这不是因为超时,因为这发生在最小的请求上,并不总是发生在大请求上.

当我删除using语句时代码正常工作:

public async Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
{
    try
    {
        HttpResponseMessage response;
        var httpClient = new HttpClient();

        response = new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = new System.Net.Http.StreamContent(
                                await httpClient.GetStreamAsync(this.filePath))
        };
        return response;
    }
    catch (WebException exception)
    {...}
}

知道为什么使用导致了这个问题吗？

Answer 1

我知道如果在异步调用完成之前处理了HttpClient,Task的状态将变为取消.但是因为我使用了await:Content = new System.Net.Http.StreamContent(等待httpClient.GetStreamAsync(this.filePath)),它应该阻止HttpClient在任务完成过程中被处理掉.

但是这项任务做了什么？它得到了流.因此,您的代码最终Stream会以关闭时可能会或可能不会完全读取的代码结束HttpClient.

HttpClient是专门为重用(和同时使用)而设计的,所以我建议using完全删除并将HttpClient声明移动到static类成员.但是如果你想关闭并重新打开客户端,你应该能够通过阅读来使它工作在关闭之前将流完全入内存HttpClient.

Answer 2

我遇到了类似于Task Canceled异常的问题.如果你尝试捕获AggregateException或捕获你Exception下面的所有块WebException,你可能会发现你捕获它,但有一个例外,条目是"任务被取消"

我做了一些调查,发现它AggregateException在各种线索中都有误导性;

将HttpClient设置为太短的超时会导致进程崩溃

我怎么知道HttpClient何时超时？

httpclientgetasync中的错误应该抛出webexception而不是taskcanceledexception

我最终更改了我的代码以设置显式超时(asyncTimeoutInMins从app.config文件中读取);

        string jsonResponse = string.Empty;
        try
        {
            using (HttpClient httpClient = new HttpClient())
            {
                httpClient.BaseAddress = new Uri(Properties.Settings.Default.MyWebService);
                httpClient.DefaultRequestHeaders.Accept.Clear();
                httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                httpClient.Timeout = new TimeSpan(0, asyncTimeoutInMins, 0);

                HttpResponseMessage response;

                response = await httpClient.GetAsync("/myservice/resource");

                // Check the response StatusCode
                if (response.IsSuccessStatusCode)
                {
                    // Read the content of the response into a string
                    jsonResponse = await response.Content.ReadAsStringAsync();
                }
                else if (response.StatusCode == HttpStatusCode.Forbidden)
                {
                    jsonResponse = await response.Content.ReadAsStringAsync();

                    Logger.Instance.Warning(new HttpRequestException(string.Format("The response StatusCode was {0} - {1}", response.StatusCode.ToString(), jsonResponse)));

                    Environment.Exit((int)ExitCodes.Unauthorised);
                }
                else
                {
                    jsonResponse = await response.Content.ReadAsStringAsync();

                    Logger.Instance.Warning(new HttpRequestException(string.Format("The response StatusCode was {0} - {1}", response.StatusCode.ToString(), jsonResponse)));

                    Environment.Exit((int)ExitCodes.ApplicationError);
                }
           }

        }
        catch (HttpRequestException reqEx)
        {
            Logger.Instance.Error(reqEx);

            Console.WriteLine("HttpRequestException : {0}", reqEx.InnerException.Message);

            Environment.Exit((int)ExitCodes.ApplicationError);
        }
        catch (Exception ex)
        {
            Logger.Instance.Error(ex);

            throw;
        }

        return jsonResponse;