Exception handling the right way for WebClient.DownloadString

Question

I was wondering what exceptions I should protect myself against when using WebClient.DownloadString.

Here's how I'm currently using it, but I'm sure you guys can suggest better more robust exception handling.

For example, off the top of my head:

No internet connection.
Server returned a 404.
Server timed out.

What is the preferred way to handle these cases and throw the exception to the UI?

public IEnumerable<Game> FindUpcomingGamesByPlatform(string platform)
{
    string html;
    using (WebClient client = new WebClient())
    {
        try
        {
            html = client.DownloadString(GetPlatformUrl(platform));
        }
        catch (WebException e)
        {
            //How do I capture this from the UI to show the error in a message box?
            throw e;
        }
    }

    string relevantHtml = "<tr>" + GetHtmlFromThisYear(html);
    string[] separator = new string[] { "<tr>" };
    string[] individualGamesHtml = relevantHtml.Split(separator, StringSplitOptions.None);

    return ParseGames(individualGamesHtml);           
}

Thomas Levesque · Accepted Answer · 2011-05-14T01:26:40.353

21

If you catch WebException, it should handle most cases. WebClient and HttpWebRequest throw a WebException for all HTTP protocol errors (4xx and 5xx), and also for network level errors (disconnection, host not reachable, etc)

How do I capture this from the UI to show the error in a message box?

I'm not sure I understand your question... Can't you just show the exception message?

MessageBox.Show(e.Message);

Don't catch the exception in FindUpcomingGamesByPlatform, let it bubble up to the calling method, catch it there and show the message...

edited May 14 '11 at 01:26

answered May 14 '11 at 01:18

Thomas Levesque

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Thanks Thomas, I guess I was asking how to bubble the exception error. :) – Only Bolivian Here May 14 '11 at 01:33

score 9 · Answer 2 · answered Mar 14 '19 at 14:27

I usually handle it like this to print any exception message the remote server is returning. Given that the users are allowed to see that value.

try
{
    getResult = client.DownloadString(address);
}
catch (WebException ex)
{
    String responseFromServer = ex.Message.ToString() + " ";
    if (ex.Response != null)
    {
        using (WebResponse response = ex.Response)
        {
            Stream dataRs = response.GetResponseStream();
            using (StreamReader reader = new StreamReader(dataRs))
            {
                responseFromServer += reader.ReadToEnd();
            }
        }
    }
    _log.Error("Server Response: " + responseFromServer);
    MessageBox.Show(responseFromServer);
}

score 5 · Answer 3 · edited Jun 04 '12 at 04:52

I use this code:

Here I init the webclient whithin the loaded event

private void LayoutRoot_Loaded(object sender, RoutedEventArgs e)
{
  // download from web async
  var client = new WebClient();
  client.DownloadStringCompleted += client_DownloadStringCompleted;
  client.DownloadStringAsync(new Uri("http://whateveraurisingis.com"));
}

The callback

void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
  #region handle download error
  string download = null;
  try
  {
    download = e.Result;
  }
catch (Exception ex)
  {
    MessageBox.Show(AppMessages.CONNECTION_ERROR_TEXT, AppMessages.CONNECTION_ERROR, MessageBoxButton.OK);
  }

  // check if download was successful
  if (download == null)
  {
    return;
  }
  #endregion

  // in my example I parse a xml-documend downloaded above      
  // parse downloaded xml-document
  var dataDoc = XDocument.Load(new StringReader(download));

  //... your code
}

Thanks.

score 1 · Answer 4 · answered May 14 '11 at 01:19

1

According to the MSDN documentation, the only non-programmer exception is WebException, which can be raised if:

The URI formed by combining BaseAddress and address is invalid.

-or-

An error occurred while downloading the resource.

answered May 14 '11 at 01:19

Jacob

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Exception handling the right way for WebClient.DownloadString

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