Weird incident with DownloadStringAsync - wpf

Question

I'm using the following code to download the source code of an HTML page
String search = $"<search url>";
String result = "";
using (WebClient wc = new WebClient())
{
var searchURL = new Uri(search);
wc.DownloadStringCompleted += (s, e) =>
{
result = e.Result;
};
wc.DownloadStringAsync(searchURL);
}
return result.Trim();
This exists in a static method
public static Scrape(String arg)
of a class Scraper.
However, when I do
txtResult.Text = Scraper.Scrape(arg);
in the code behing of a form, nothing appears. Using breakpoints, I see that e.Result indeed contains the expected data, but something happens and the data is "lost" in the way. Anyone has any idea what I'm missing;

Answer 1

Because the download is asynchronous it hasn't updated the value of result before you reach the line return "result.Trim()" as a result you are returning the empty string.
You need to wait for WC to complete, if you use the task method then accessing the result property will cause you to wait for the response.
String search = $"<search url>";
String result = "";
using (WebClient wc = new WebClient())
{
var searchURL = new Uri(search);
result = wc.DownloadStringTaskAsync(searchURL).Result;
}
return result.Trim();
However if you do this in the current method as it stands you will end up hanging your UI as the wait happens on the foreground thread.
You can move the update to the background by changing your call to use a task with a continuation.
Instead of
txtResult.Text = Scraper.Scrape(arg);
Add a using using System.Threading.Tasks;
then you can use the line
Task.Run(() => Scraper.Scrape(arg))
.ContinueWith(t => txtResult.Text = t.Result,
TaskScheduler.FromCurrentSynchronizationContext());
You may also want to precede this with a line txtResult.Text = "Please Wait Fetching....";