C# Download File Asynchronously from winform

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using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Text; using System.Windows.Forms; using System.Net;   namespace WindowsTreeViewDemo {     /// <summary>     /// 2011-06-25     /// 涂聚文  QQ天气预报     /// </summary>     public partial class Form1 : Form     {         WebClient webClient = new WebClient();           public Form1()         {             InitializeComponent();         }           private void Form1_Load(object sender, EventArgs e)         {           }         /// <summary>         ///         /// </summary>         /// <param name="sender"></param>         /// <param name="e"></param>         private void button1_Click(object sender, EventArgs e)         {                           webClient.DownloadFileCompleted += new AsyncCompletedEventHandler(Completed);             webClient.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged);             webClient.DownloadFileAsync(new Uri("http://weather.qq.com/inc/ss1.htm"), @"c:\201106021ss1.html");           }         /// <summary>         ///         /// </summary>         /// <param name="sender"></param>         /// <param name="e"></param>         private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e)         {             progressBar1.Value = e.ProgressPercentage;             this.label1.Text = "已完成：" + progressBar1.Value.ToString() + "%";           }         /// <summary>         ///         /// </summary>         /// <param name="sender"></param>         /// <param name="e"></param>         private void Completed(object sender, AsyncCompletedEventArgs e)         {             MessageBox.Show("Download completed!");         }         /// <summary>         ///         /// </summary>         /// <param name="sender"></param>         /// <param name="e"></param>         private void button2_Click(object sender, EventArgs e)         {             //取消上传             if (webClient != null)             {                 webClient.CancelAsync();             }             Close();           }         } }