Exception handling the right way for WebClient.Dow

问题:

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);           
}

回答1:

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...

回答2:

I use this code:

1. 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"));
   }
   

2. 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.

回答3:

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.