asp.net WinForm下载文件并显示下载进度条教程

/// <summary>
/// 下载文件
/// </summary>
/// <param name="url"></param>
/// <param name="savefile"></param>
/// <param name="downloadProgressChanged"></param>
/// <param name="downloadFileCompleted"></param>
private void DownloadFile(string url, string savefile, Action<int> downloadProgressChanged, Action downloadFileCompleted)
{
    WebClient client = new WebClient();
    if (downloadProgressChanged != null)
    {
        client.DownloadProgressChanged += delegate(object sender, DownloadProgressChangedEventArgs e)
        {
            this.Invoke(downloadProgressChanged, e.ProgressPercentage);
        };
    }
    if (downloadFileCompleted != null)
    {
        client.DownloadFileCompleted += delegate(object sender, AsyncCompletedEventArgs e)
        {
            this.Invoke(downloadFileCompleted);
        };
    }
    client.DownloadFileAsync(new Uri(url), savefile);
}
delegate void Action(); //.NET Framework 2.0得自定义委托Action

/// <summary>
/// 点击下载
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void button1_Click(object sender, EventArgs e)
{
    DownloadFile("http://www.111cn.net/ update.zip", @"F:update.zip", ProgressBar_Value, null);
}