How to use the WebClient.DownloadDataAsync() method in this context?

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.

Re IMDB: terms page: "Robots and Screen Scraping: You may not use data mining, robots, screen scraping, or similar data gathering and extraction tools on this site, except with our express written consent as noted below.". I **strongly suggest** that you do **not** this; it is clearly against their rules. — Marc Gravell, Oct 18 '09 at 20:53
Marc, what other site has information like IMDB that I CAN use? Thanks for the help. — Sergio Tapia, Oct 18 '09 at 21:22
@SergioTapia also, themoviedb.org and thetvdb.com which I believe both have APIs you can use to access the data directly and save yourself alot of time. ;) — Arvo Bowen, Feb 24 '16 at 16:38

score 40 · Answer 1 · edited Feb 03 '15 at 15:55

40

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);

edited Feb 03 '15 at 15:55

Jason Evans

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answered Aug 07 '14 at 06:18

Bill Forney

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But do this is not the same to do the call in a sync way? (It is DownloadDataTask and in this case you will receive the result directly), if you want to do an async call I think this is not an option, because is working like sync. – Daniel Silva Nov 22 '17 at 12:41
1

It uses async/await with the Task class. It is asynchronous as much as the above answer is. The only difference is you use a continuation method DownloadDataCompleted in the above and you use an await to inline the continuation in my version. If you wanted progress reporting or the like I might do it differently but if you just want a continuation on completion this way works just as well with less code. – Bill Forney Nov 23 '17 at 23:19
2

If you want to do other things while the Task is running you just don't await it and do your stuff, then add a continuation to it. [Check this out for info](https://learn.microsoft.com/en-us/dotnet/standard/parallel-programming/chaining-tasks-by-using-continuation-tasks). – Bill Forney Nov 23 '17 at 23:25

score 31 · Answer 2 · answered Oct 18 '09 at 20:49

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.

Just to dot-the-i, there is a standard pattern for handling `DownloadDataCompleted`, like for `RunWorkerCompleted`, see http://msdn.microsoft.com/en-us/library/cc221403%28VS.95%29.aspx — H H, Oct 18 '09 at 21:47
as Bill Forney said, DownloadDataTaskAsync is a better solution — hvojdani, Sep 26 '19 at 12:49

score 5 · Answer 3 · edited Jan 15 '17 at 18:46

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);
}

score 3 · Answer 4 · answered Nov 30 '12 at 10:29

3

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

answered Nov 30 '12 at 10:29

ssmsnet

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score 1 · Answer 5 · edited Oct 19 '16 at 15:13

1

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

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

edited Oct 19 '16 at 15:13

Roman Marusyk

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answered Oct 19 '16 at 15:08

OnicDr

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1

score 0 · Answer 6 · answered Aug 11 '16 at 00:57

//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);
}

How to use the WebClient.DownloadDataAsync() method in this context?

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