C# 如何使用Youtube获取视频信息

我正在尝试开发一个Youtube下载程序。我需要视频mp4流URL，我正在尝试使用get_视频信息检查我的源代码

WebRequest req = HttpWebRequest.Create("https://www.youtube.com/get_video_info?video_id=" + videoid +"&el=embedded&ps=default&eurl=&gl=US&hl=en");
            WebResponse webResp = req.GetResponse();


        StreamReader sr = new StreamReader(webResp.GetResponseStream(), System.Text.Encoding.UTF8);
            string data = sr.ReadToEnd();
            sr.Close();
            webResp.Close();

这种代码总是只得到一部分数据，可能是一半。解码后的数据就是这样

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using (var client = new HttpClient()) {
    var response = await client.GetAsync(url);
    string content = await response.Content.ReadAsStringAsync();
}