Would this Asynchronous download work? WebClient.DownloadDataAsyn() problem.

Question

I have this class called SiteAsyncDownload.cs

Here's the code:

public class SiteAsyncDownloader
{
    WebClient Client = new WebClient();
    string SiteSource = null;

    /// <summary>
    /// Download asynchronously the source code of any site in string format.
    /// </summary>
    /// <param name="URL">Site URL to download.</param>
    /// <returns>Website source code - string.</returns>
    public string GetSite(string URL)
    {
        Uri Site = new Uri(URL);

        Client.DownloadDataAsync(Site);
        Client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(FinishedDownloading);

        //Don't know if this would work. Consult with StackOverflow.
        while (true)
        {
            if (SiteSource != null)
            {
                return SiteSource;
            }
        }            
    }

    void FinishedDownloading(object sender, DownloadDataCompletedEventArgs e)
    {
        SiteSource = Encoding.ASCII.GetString(e.Result);
        throw new NotImplementedException();
    }
}

I'm not 100% if this would work as I want it to. I want the class to download whatever it needs to asynchronously and when done downloading return the string. Is this the right approach?

For instance, here's an example of how I intend to use it:

SiteAsyncDownloader Downloader = new SiteAsyncDownloader();

    /// <summary>
    /// Search for a movie using only a name.
    /// </summary>
    /// <param name="MovieName">Movie name.</param>        
    public void SearchMovie(string MovieName)
    {
        string SearchURL = FormatSearch(MovieName);

        string SearchSource = Downloader.GetSite(SearchURL);

        string MovieURL = FindMovieURL(SearchSource);
        string MovieSource = Downloader.GetSite(MovieURL);

        FindInformation(MovieSource);
    }

In the second line of code in the SearchMovie() method, will my program crash because it's of the async download? How can I take this into account and have it work properly?

Answer 1

While it will probably work, it's a bad idea. There's no point in using asynchronous APIs if you're immediately going to block until the results have come through. If you really can't do any more work in that thread until you've got the data, and you've only got one request, just use a synchronous API. Further, the way you're blocking is to just tight loop, which will cause your processor to spin for no reason.

On a slightly different matter, I wouldn't use Encoding.ASCII if I were you - you should honour whatever encoding the server sends. Using DownloadStringAsync is the easiest way of doing that.

Answer 2

What you're trying to do here:

while (true)

Will just spin up the CPU until the operation is done, defeating the point of it being asynchronous in the first place. You would need to use the event pattern or the callback pattern.