c＃WebClient DownloadFileAsync（）不会抛出错误

Question

我正在使用以下代码成功下载文件。因为我希望它们在Application Startup上下载，所以我不想阻止任何东西 - > async。

但是我面临的问题是，即使URI路径完全无意义，它也会生成一个空文件，而不是像msdn中所述那样引发错误

谁能帮我？

private void DownloadDocuments()
    {
        using (WebClient myWebClient = new WebClient())
        {
            try
            {
                Log("load1");
                myWebClient.DownloadFileAsync(new Uri(documentsUri + documentActivation), documentspath + "\\" + documentActivation);

            }
            catch (WebException)
            {
                Log("Guide Activation Download failed.");
            }
            catch (InvalidOperationException)
            {
                Log("Guide Activation could not be saved.");
            }
        }
        using (WebClient myWebClient = new WebClient())
        { 
            try
            {
                Log("load2");
                myWebClient.DownloadFileAsync(new Uri(documentsUri + documentFloating), documentspath + "\\" + documentFloating);
            }
            catch (WebException)
            {
                Log("Guide Floating Download failed.");
            }
            catch (InvalidOperationException)
            {
                Log("Guide Floating could not be saved.");
            }
        }
        using (WebClient myWebClient = new WebClient())
        {
            try
            {
                Log("load3");
                myWebClient.DownloadFileAsync(new Uri(documentsUri + documentSeat), documentspath + "\\" + documentSeat);
            }
            catch (WebException)
            {
                Log("Guide Seat Download failed.");
            }
            catch (InvalidOperationException)
            {
                Log("Guide Seat could not be saved.");
            }
        }
    }

Answer 1

WebClient.DownloadFileAsync不会在HTTP请求失败时抛出异常。您需要订阅DownloadFileCompleted事件以获得错误通知。

但是，一旦我们在C＃中具有基于任务的异步/等待功能，我不建议搞乱事件处理程序回调：WebClient.DownloadFileTaskAsync使用起来更方便。

考虑到有关并行处理的注释，您可以执行以下操作：

static async Task DownloadDocumentAsync(Uri uri, string fileName)
{
    using (var webClient = new WebClient())
    {
        try
        {
            await webClient.DownloadFileTaskAsync(uri, fileName);
        }
        catch (WebException ex)
        {
            Log($"Downloading {uri} failed. {ex.Message}");
            throw;
        }
        catch (InvalidOperationException)
        {
            Log($"Saving {uri} to {fileName} failed. File is in use.");
            throw;
        }
    }
}

然后你的应用程序启动逻辑：

var baseUri = new Uri(documentsUri);
var downloadTasks = new[]
{
    DownloadDocumentAsync(new Uri(baseUri, documentActivation), Path.Combine(documentspath, documentActivation)),
    DownloadDocumentAsync(new Uri(baseUri, documentFloating), Path.Combine(documentspath, documentFloating)),
    DownloadDocumentAsync(new Uri(baseUri, documentSeat), Path.Combine(documentspath, documentSeat)),
};

try
{
    Task.WaitAll(downloadTasks);
}
catch (AggregateException)
{
    // handle the case when some of the download tasks failed
}

这样下载任务就会并行执行，但Task.WaitAll会阻塞，直到完成所有任务。如果你想保持异步，你需要await Task.WhenAll。