WebClient异步下载文件

 private WebClient MyWebClient = new WebClient();
        private void MyWebClient_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message, "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            else if ((e.Cancelled == true))
            {
                MessageBox.Show("下载文件操作被取消！", "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            else
            {
                MessageBox.Show("下载文件操作完成！", "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }
        private void MyWebClient_DownloadProgressChanged(object sender, System.Net.DownloadProgressChangedEventArgs e)
        {
            this.progressBar1.Value = e.ProgressPercentage;
            this.label3.Text = "已经下载" + e.BytesReceived.ToString() + " 字节，全部共有" + e.TotalBytesToReceive.ToString()+"字节";
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            MyWebClient.DownloadFileCompleted += new AsyncCompletedEventHandler(MyWebClient_DownloadFileCompleted);
            MyWebClient.DownloadProgressChanged += new DownloadProgressChangedEventHandler(MyWebClient_DownloadProgressChanged);
        }
        //开始下载
        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                this.progressBar1.Value = 0;
                this.MyWebClient.DownloadFileAsync(new Uri(this.textBox1.Text), this.textBox2.Text);
            }
            catch (Exception MyEx)
            {
                MessageBox.Show(MyEx.Message, "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }
        //取消下载
        private void button2_Click(object sender, EventArgs e)
        {
            MyWebClient.CancelAsync();
        }