//代码：

using System;

using System.ComponentModel;

using System.Net;

using System.Threading;

using System.Windows.Forms;

namespace DownFile

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

        }

        string netPath = "http://wlkt.zufe.edu.cn/Skyclass/C91/Courseware/Bbs/080717071122.mp3";//需要下载的文件

        string filePath = "D:\\just one last dance.mp3";//文件保存的路径

        delegate void delDownFileHandler(int totalNum, int num, int proc);//用于指示文件信息（文件大小，当前下载的数量，当前下载的百分比）

        delegate void delComDownFileHandler(bool isCompleted);//文件下载完成之后

        WebClient wc = new WebClient();

        private void button1_Click(object sender, EventArgs e)

        {

            wc.DownloadProgressChanged += new DownloadProgressChangedEventHandler(wc_DownloadProgressChanged);

            wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileCompleted);

            progressBar1.Value = ;

            label1.Text = "";

            button2.Enabled = true;

            button1.Enabled = false;

            //使用线程启动

            Thread td = new Thread(new ThreadStart(loadFile));

            td.IsBackground = true;

            td.Start();

        }

        //线程启动

        private void loadFile()

        {

            if (wc.IsBusy)

            {

                wc.CancelAsync();

            }

            wc.DownloadFileAsync(new Uri(netPath), filePath);

        }

         //更新过程中触发的事件（在线程中不能直接操纵控件，因此用委托执行）

        void wc_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)

        {

            this.Invoke(new delDownFileHandler(processShow), new object[] { (int)e.TotalBytesToReceive, (int)e.BytesReceived, e.ProgressPercentage });

        }

        //委托执行  （进度条和字符标识）

        private void processShow(int totalNum, int num, int proc)

        {

            progressBar1.Maximum = totalNum;

            progressBar1.Value = num;

            label1.Text = "正在下载：" + (num / ()).ToString() + "KB/" + (totalNum / ).ToString() + "KB   " + proc.ToString() + "%";

        }

        //完成更新之后触发的事件

        void wc_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)

        {

            if (e.Cancelled)

                this.Invoke(new delComDownFileHandler(comDownFile), new object[] { false });

            else

                this.Invoke(new delComDownFileHandler(comDownFile), new object[] { true });

        }

        private void comDownFile(bool isCompleted)

        {

            if (isCompleted)

                label1.Text = "下载完成";

            else

                label1.Text = "下载取消";

            progressBar1.Value = ;

            button1.Enabled = true;

            button2.Enabled = false;

        }

        //点击取消后

        private void button2_Click(object sender, EventArgs e)

        {

            wc.CancelAsync();

            wc.Dispose();

        }

    }

}