Question

Here is the function I have now that obviously doesn’t work. The reason it doesn’t work is because WebClient is asynchronous and data is empty before it gets filled by WebClient and crashes on the XML reader. How can I call WebClient within this function and still allow it to return ServerResult as required with or without needing an external event handler?

static public ServerResult isBarcodeCorrectOnServer(string barcode)
{
            Dictionary<string, IPropertyListItem> dict = configDictionary();

            string urlString = (string.Format("http://www.myurl.com/app/getbarcodetype.php?realbarcode={0}&type={1}", barcode, dict["type"]));

            WebClient wc = new WebClient();
            string data = "";
            wc.DownloadStringCompleted += (sender, e) =>
            {
                if (e.Error == null)
                {
                    //Process the result...
                    data = e.Result;
                }
            };
            wc.DownloadStringAsync(new Uri(urlString));

            StringReader stream = new StringReader(data);
            var reader = XmlReader.Create(stream);
            var document = XDocument.Load(reader);
            var username = document.Descendants("item");
            var theDict = username.Elements().ToDictionary(ev => ev.Name.LocalName, ev => ev.Value);

            if (theDict.ContainsKey("type") == true && theDict["type"].ToString() == dict["type"].ToString())
            {
                return ServerResult.kOnServer;
            }
            else if (theDict.ContainsKey("type") == true)
            {
                return ServerResult.kWrongType;
            }
            else
            {
                return ServerResult.kNotOnServer;
            }
        }

Answer 1

You can’t without “hacks” and you shouldn’t – embrace asynchrony and pass in a delegate that you want to be executed once the download is completed:

static public void isBarcodeCorrectOnServer(string barcode, Action<string> completed)
{
    //..
    wc.DownloadStringCompleted += (sender, e) =>
    {
       if (e.Error == null)
       {
            //Process the result...
            data = e.Result;
            completed(data);
       }
    };
    //..
}

You can move all your processing code now into a separate method which you call with the download result.