C# 如何在这种情况下使用 WebClient.DownloadDataAsync() 方法？

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

提问by Sergio Tapia

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.

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，因此 UI 不会冻结。

Here's the code:

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;
        }

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?

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站点的字节（为了以后使用，我会将其转换为字符串，我知道如何执行此操作）并且不会冻结我的 GUI？

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

也许是 DownloadDataAsync 的一个清晰示例，以便我可以学习如何使用它？

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

谢谢，你总是这么棒的资源。

回答by Marc Gravell

You need to handle the DownloadDataCompletedevent:

您需要处理该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");
}

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

args 包含与错误条件等相关的其他信息位 - 也请检查这些信息。

Also note that you'll be coming into DownloadDataCompletedon 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.

另请注意，您将进入DownloadDataCompleted不同的话题；如果您在 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.

回答by Rubens Farias

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);
}

回答by ssmsnet

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

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"。

回答by Bill Forney

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...

有一个较新的 DownloadDataTaskAsync 方法可以让您等待结果。到目前为止，它更易于阅读且更易于连接。我会用那个...

var client = new WebClient();

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

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

回答by Hung Le

//using ManualResetEvent class

//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);
}

回答by OnicDr

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

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

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