Trouble with WebClient.DownloadDataAsync(). Not downloading the data?

问题

public string[] SearchForMovie(string SearchParameter) {
WebClientX.DownloadDataCompleted += new DownloadDataCompletedEventHandler(WebClientX_DownloadDataCompleted); WebClientX.DownloadDataAsync(new Uri( "http://www.imdb.com/find?s=all&q=ironman+&x=0&y=0"));

    string sitesearchSource = Encoding.ASCII.GetString(Buffer);
}

void WebClientX_DownloadDataCompleted(object sender,
    DownloadDataCompletedEventArgs e)
{
    Buffer = e.Result;
    throw new NotImplementedException();
}

I get this exception:

The matrix cannot be null. Refering to my byte[] variable Buffer.

So, I can conclude that the DownloadDataAsync isn't really downloading anything. What is causing this problem?

PS. How can I easily format my code so it appear properly indented here. Why can't I just copy past the code from Visual C# express and maintain the indentation here? Thanks! :D

回答1:

The key word here is "async"; when you call DownloadDataAsync, it only starts the download; it isn't complete yet. You need to process the data in the callback (WebClientX_DownloadDataCompleted).

public string[] SearchForMovie(string SearchParameter)
{
    WebClientX.DownloadDataCompleted += WebClientX_DownloadDataCompleted;
    WebClientX.DownloadDataAsync(new Uri(uri));
}

void WebClientX_DownloadDataCompleted(object sender,
     DownloadDataCompletedEventArgs e)
{
    Buffer = e.Result;
    string sitesearchSource = Encoding.ASCII.GetString(Buffer);
}

Also - don't assume ASCII; WebClientX.Encoding would be better; or just DownloadStringAsync:

static void Main()
{
    var client = new WebClient();
    client.DownloadStringCompleted += DownloadStringCompleted;
    client.DownloadStringAsync(new Uri("http://google.com"));
    Console.ReadLine();
}

static void DownloadStringCompleted(object sender,
    DownloadStringCompletedEventArgs e)
{
    if (e.Error == null && !e.Cancelled)
    {
        Console.WriteLine(e.Result);
    }
}

来源：https://stackoverflow.com/questions/1586184/trouble-with-webclient-downloaddataasync-not-downloading-the-data

标签

winforms

asynchronous

webclient