如何在此上下文中使用 WebClient.DownloadDataAsync() 方法？

Question

我的计划是让用户在我的程序中写下电影标题，我的程序将异步提取适当的信息，这样 UI 就不会冻结。

这是代码：

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;
        }

我的实际问题在于 WebClientX.DownloadDataAsync() 方法。我不能使用字符串 URL。我如何使用该内置函数来下载网站的字节（为了以后使用，我会将其转换为字符串，我知道如何做到这一点）而不冻结我的 GUI？

也许是 DownloadDataAsync 的清晰示例，以便我可以学习如何使用它？

谢谢你，你总是一个很棒的资源。

Answer 1

有一个较新的 DownloadDataTaskAsync 方法可让您等待结果。到目前为止，它更容易阅读并且更容易连接。我会用那个...

var client = new WebClient();

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

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

Answer 2

您需要处理 DownloadDataCompleted 事件：

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");
}

args 包含与错误条件等相关的其他信息 - 也请检查这些信息。

另请注意，您将在不同的线程上进入 DownloadDataCompleted；如果您处于 UI（winform、wpf 等）中，则需要在更新 UI 之前进入 UI 线程。在 winforms 中，使用 this.Invoke。对于 WPF，请查看Dispatcher。

Answer 3

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 4

如果有人在 Web 应用程序或网站中使用上述内容，请在 aspx 文件的页面指令声明中设置 Async = "true"。

Answer 5

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

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

Answer 6

//使用ManualResetEvent类

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);
}