c# - 中止 GetResponseAsync() 和任务中的异常

Question

我写了一个方法，它从 eq 中获取 ETag。服务器上的 XML 文件。如果超时(GetResponseAsync() 没有CancellationToken)，我是否正确编写了中止任务并且我不知道如何执行Exception。

代码如下:

    public static async Task<string> GetETagAsync(Uri feedLink)
    {
        const int millisecondsTimeout = 2500;
        WebRequest webRequest = WebRequest.Create(feedLink);
        webRequest.Method = "HEAD";
        try
        {
            Task<WebResponse> webResponse = webRequest.GetResponseAsync();
            if (await Task.WhenAny(webResponse, Task.Delay(millisecondsTimeout)) == webResponse)
            {
                using (var result = webResponse.Result)
                {
                    return result.Headers["ETag"];
                }
            }
            else
            {
                webRequest.Abort();
                return null;
            }
        }
        catch (Exception)
        {
            return null;
        }
    }

---

编辑

我做了一些改变。重写异常并使用本主题中的类:GetResponseAsync does not accept cancellationToken

代码:

    public static async Task<string> GetETagAsync(Uri feedLink)
    {
        const int millisecondsTimeout = 2500;
        var cancellationTokenSource = new CancellationTokenSource();
        WebRequest webRequest = WebRequest.Create(feedLink);
        webRequest.Method = "HEAD";

        try
        {
            Task<WebResponse> webResponse = WebRequestExtensions.GetResponseAsync(webRequest, cancellationTokenSource.Token);
            if (await Task.WhenAny(webResponse, Task.Delay(millisecondsTimeout)) == webResponse)
            {
                using (var result = webResponse.Result)
                {
                    return result.Headers["ETag"];
                }
            }
            else
            {
                cancellationTokenSource.Cancel();
                return null;
            }
        }
        catch (AggregateException ex)
        {
            if (ex.InnerException is WebException)
                return null;

            throw;
        }
    }

public static class WebRequestExtensions
{
    public static async Task<WebResponse> GetResponseAsync(this WebRequest request, CancellationToken cancellationToken)
    {
        using (cancellationToken.Register(() => request.Abort(), useSynchronizationContext: false))
        {
            try
            {
                var response = await request.GetResponseAsync();
                cancellationToken.ThrowIfCancellationRequested();
                return (WebResponse)response;
            }
            catch (WebException ex)
            {
                if (ex.Status == WebExceptionStatus.RequestCanceled)
                {
                    cancellationToken.ThrowIfCancellationRequested();
                }

                if (cancellationToken.IsCancellationRequested)
                {
                    throw new TaskCanceledException(ex.Message, ex);
                }

                throw;
            }
        }
    }
}

现在正确吗？

Answer 1

您可以使用我在此处编写的WebRequestExtensions 类来解决问题: https://github.com/openstacknetsdk/openstack.net/blob/master/src/corelib/Core/WebRequestExtensions.cs

此类完全支持 CancellationToken 和 WebRequest.Timeout 属性。