Question

我对如何做到这一点非常困惑。

我有一堆后台工作人员产生，所有人都需要使用Webclients downloadstringasync方法。

我的问题是如何从下载的字符串返回数据（一旦下载字符串完成事件发生），回到启动downloadstring的后台工作者？

我希望这是有道理的。任何建议将不胜感激。

Answer 1

在工作线程中使用异步版本毫无意义。由于BackgroundWorker，代码已经异步运行。

所以只需使用DownloadString（），问题就解决了。

Answer 2

如果使用WebClient.DownloadStringAsync，它将在不同的线程上运行。您需要添加事件处理程序以在事件发生时收到通知。

例如：

WebClient.DownloadStringCompleted Event

此外，以下是一些从Google Async下载图片的示例代码。

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Threading;

namespace WebDownload
{
    class Program
    {
        static AutoResetEvent autoEvent = new AutoResetEvent(false);

        static void Main(string[] args)
        {
            WebClient client = new WebClient();
            client.DownloadFileCompleted += new AsyncCompletedEventHandler(DownloadCompleted);

            string fileName = Path.GetTempFileName();
            client.DownloadFileAsync(new Uri("https://www.google.com/images/srpr/logo11w.png"), fileName, fileName);

            // Wait for work method to signal. 
            if (autoEvent.WaitOne(20000))
                Console.WriteLine("Image download completed.");
            else
            {
                Console.WriteLine("Timed out waiting for image to download.");
                client.CancelAsync();
            }
        }

        private static void DownloadCompleted(object sender, AsyncCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                Console.WriteLine(e.Error.Message);
                if (e.Error.InnerException != null)
                    Console.WriteLine(e.Error.InnerException.Message);
            }
            else
            {
                // We have a file - do something with it
                WebClient client = (WebClient)sender;

                // display the response header so we can learn
                Console.WriteLine("Response Header...");
                foreach(string k in client.ResponseHeaders.AllKeys)
                    Console.WriteLine("   {0}: {1}", k, client.ResponseHeaders[k]);
                Console.WriteLine(string.Empty);

                // since we know it's a png, let rename it
                FileInfo temp = new FileInfo((string)e.UserState);
                string pngFileName = Path.Combine(Path.GetTempPath(), "DesktopPhoto.png");
                if (File.Exists(pngFileName))
                    File.Delete(pngFileName);

                File.Move((string)e.UserState, pngFileName);  // move to where ever you want
                Process.Start(pngFileName);     // display the photo for the fun of it
            }

            // Signal that work is finished.
            autoEvent.Set();
        }


    }
}

在Backgroundworker vb.net中的DownloadStringAsync

2 个答案: