How to use the WebClient.DownloadDataAsync() metho

Question

My plan is to have a user write down a movie title in my program and my program will pull the appropiate information asynchronously so the UI doesn't freeze up.

Here's the code:

public class IMDB
    {
        WebClient WebClientX = new WebClient();
        byte[] Buffer = null;


        public string[] SearchForMovie(string SearchParameter)
        {
            //Format the search parameter so it forms a valid IMDB *SEARCH* url.
            //From within the search website we're going to pull the actual movie
            //link.
            string sitesearchURL = FindURL(SearchParameter);

            //Have a method download asynchronously the ENTIRE source code of the
            //IMDB *search* website.
            Buffer = WebClientX.DownloadDataAsync(sitesearchURL);


            //Pass the IMDB source code to method findInformation().

            //string [] lol = findInformation();

            //????

            //Profit.

            string[] lol = null;
            return lol;
        }

My actual problem lies in the WebClientX.DownloadDataAsync() method. I can't use a string URL for it. How can I use that built in function to download the bytes of the site (for later use I will convert this to string, I know how to do this) and without freezing up my GUI?

Perhaps a clear cut example of the DownloadDataAsync so I can learn how to use it?

Thanks SO, you're always such a great resource.

Answer 1

ThreadPool.QueueUserWorkItem(state => WebClientX.DownloadDataAsync(sitesearchURL));

http://workblog.pilin.name/2009/02/system.html

Answer 2

There is a newer DownloadDataTaskAsync method that allows you to await the result. It is simpler to read and easier to wire up by far. I'd use that...

var client = new WebClient();

var data = await client.DownloadDataTaskAsync(new Uri(imageUrl));

await outstream.WriteAsync(data, 0, data.Length);

Answer 3

//using ManualResetEvent class

static ManualResetEvent evnts = new ManualResetEvent(false);
static void Main(string[] args)
{
    byte[] data = null;
    WebClient client = new WebClient();
    client.DownloadDataCompleted += 
        delegate(object sender, DownloadDataCompletedEventArgs e)
        {
             data = e.Result;
             evnts.Set();
        };
    Console.WriteLine("starting...");
    evnts.Reset();
    client.DownloadDataAsync(new Uri("http://stackoverflow.com/questions/"));
    evnts.WaitOne(); // wait to download complete

    Console.WriteLine("done. {0} bytes received;", data.Length);
}

Answer 4

static void Main(string[] args)
{
    byte[] data = null;
    WebClient client = new WebClient();
    client.DownloadDataCompleted += 
       delegate(object sender, DownloadDataCompletedEventArgs e)
       {
            data = e.Result;
       };
    Console.WriteLine("starting...");
    client.DownloadDataAsync(new Uri("http://stackoverflow.com/questions/"));
    while (client.IsBusy)
    {
         Console.WriteLine("\twaiting...");
         Thread.Sleep(100);
    }
    Console.WriteLine("done. {0} bytes received;", data.Length);
}

Answer 5

If anyone using above in web application or websites please set Async = "true" in the page directive declaration in aspx file.

Answer 6

You need to handle the DownloadDataCompleted event:

static void Main()
{
    string url = "http://google.com";
    WebClient client = new WebClient();
    client.DownloadDataCompleted += DownloadDataCompleted;
    client.DownloadDataAsync(new Uri(url));
    Console.ReadLine();
}

static void DownloadDataCompleted(object sender,
    DownloadDataCompletedEventArgs e)
{
    byte[] raw = e.Result;
    Console.WriteLine(raw.Length + " bytes received");
}

The args contains other bits of information relating to error conditions etc - check those too.

Also note that you'll be coming into DownloadDataCompleted on a different thread; if you are in a UI (winform, wpf, etc) you'll need to get to the UI thread before updating the UI. From winforms, use this.Invoke. For WPF, look at the Dispatcher.