C#写下载器的问题

        private void button1_Click(object sender, EventArgs e)
        {
            ServicePointManager.DefaultConnectionLimit = 1024;
            MessageBox.Show(""+ ServicePointManager.DefaultConnectionLimit);
            WebClient client = new WebClient();
            client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback);
            client.DownloadFileTaskAsync("https://lon-gb-ipv6.vultr.com/vultr.com.1000MB.bin", "d:\\gb.bin");
            WebClient client2 = new WebClient();
            client2.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback2);
            client2.DownloadFileTaskAsync("https://par-fr-ipv6.vultr.com/vultr.com.1000MB.bin", "d:\\fr.bin");
            WebClient client3 = new WebClient();
            client3.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback3);
            client3.DownloadFileTaskAsync("https://fra-de-ipv6.vultr.com/vultr.com.1000MB.bin", "d:\\de.bin");
        }

        private void DownloadProgressCallback(object sender, DownloadProgressChangedEventArgs e)
        {
            progressBar1.Value = e.ProgressPercentage;
        }

        private void DownloadProgressCallback2(object sender, DownloadProgressChangedEventArgs e)
        {
            progressBar2.Value = e.ProgressPercentage;
        }

        private void DownloadProgressCallback3(object sender, DownloadProgressChangedEventArgs e)
        {
            progressBar3.Value = e.ProgressPercentage;
        }

为什么无法正常下载。。。。进度条飞快到达 100%

BTW 用 WebHttpRequest 还是用 WebClient 写下载器好？