使用异步编程捕获异常

Question

VS C#2005

我使用下面的代码使用Web客户端上传文件Async.但是,如果出现问题,我不确定是否可以捕获异常.因为wc.UpLoadFileAsync会马上回来.所以放在try...catch那里似乎毫无意义.

所以为了测试,我故意创建了一个错误,看看如果我输入错误的URL会发生什么.但是,我在调试输出窗口中得到了这个.

A first chance exception of type 'System.Net.WebException' occurred in System.dll
File completed

所以它不会发射try...catch.它仍然触发了这种UploadFileCompleted方法

private void upload_config_to_server()
{
    Uri url = new Uri("http://10.10.10.3/softphone/config.xml");

    WebClient wc = new WebClient();
    if (!wc.IsBusy)
    {
        wc.UploadProgressChanged += new UploadProgressChangedEventHandler(wc_UploadProgressChanged);
        wc.UploadFileCompleted += new UploadFileCompletedEventHandler(wc_UploadFileCompleted);      

        try
        {
            wc.UploadFileAsync(url, "PUT", "config.xml");
        }
        catch (WebException webex)
        {
            Console.WriteLine("Web Exception {0}", webex.Message);
        }
        catch (Exception ex)
        {
            Console.WriteLine("Exception {0}", ex.Message);
        }
    }
}

private void wc_UploadProgressChanged(object sender, UploadProgressChangedEventArgs e)
{
    Console.WriteLine("Bytes uploaded {0}", e.BytesSent);
}

private void wc_UploadFileCompleted(object sender, UploadFileCompletedEventArgs e)
{
    Console.WriteLine("File completed");
}  

有没有正确的方法来处理这个问题.我只想向用户显示一个消息框,以显示上传文件失败的原因.但是,由于上传的文件很大,上传文件可能需要几秒钟.所以我需要使用Async来防止UI冻结.

非常感谢任何建议,

Answer 1

您可以检查e.Error值.

private void wc_UploadFileCompleted(object sender, UploadFileCompletedEventArgs e)
{
    if (e.Error != null)
    {
        if (e.Error is WebException)
            Console.WriteLine("Web Exception {0}", ((WebException)e.Error).Message);
        else
            Console.WriteLine("Web Exception {0}", e.Error.Message);
    }
    else if (e.Cancelled)
        Console.WriteLine("File cancelled");
    else
        Console.WriteLine("File completed");
}