C# Download File Asynchronously from winform

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(); 

        }


    }
}