WebException：远程服务器返回错误：NotFound Windows Phone Youtube Html

我想在youtube搜索结果中获取元素和标签，下面是windows phone应用程序的代码 .

public partial class Page1 : PhoneApplicationPage
{
    string keyword;
    public Page1()
    {
        InitializeComponent();

    }

    protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        NavigationContext.QueryString.TryGetValue("parameter", out keyword);
        LoadResults();
    }

    public void LoadResults()
    {          
        WebClient codeSampleReq = new WebClient();
        codeSampleReq.DownloadStringCompleted += codeSampleReq_DownloadStringCompleted;
        codeSampleReq.DownloadStringAsync(new Uri("https://www.youtube.com/results?search_query=" + keyword));
    }



    void codeSampleReq_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        try
        {
            HtmlDocument htmlDoc = new HtmlAgilityPack.HtmlDocument();
            htmlDoc.OptionFixNestedTags = true;
            htmlDoc.LoadHtml(e.Result);
            HtmlNode divContainer = htmlDoc.GetElementbyId("a");
            if (divContainer != null)
            {
                HtmlNodeCollection nodes = divContainer.SelectNodes("a");
                foreach (HtmlNode trNode in nodes)
                {

                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("Unable to download" + ex.Message);
        }
    }
}

并在下面获得例外：

{System.Net.WebException：远程服务器返回错误：NotFound . ---> System.Net.WebException：远程服务器返回错误：NotFound . 在System.Net.Browser.ClientHttpWebRequest上的System.Net.Browser.ClientHttpWebRequest.InternalEndGetResponse（IAsyncResult asyncResult） . <> c__DisplayClasse.b__d（Object sendState）在System.Net.Browser.AsyncHelper . <> c__DisplayClass1.b__0（Object sendState） ---内部异常堆栈跟踪的结束---在System.Net.WebClient的System.Net.Browser.ClientHttpWebRequest.EndGetResponse（IAsyncResult asyncResult）的System.Net.Browser.AsyncHelper.BeginOnUI（SendOrPostCallback beginMethod，Object state）中 . System.Net.WebClient.DownloadBitsResponseCallback（IAsyncResult result）}中的GetWebResponse（WebRequest请求，IAsyncResult结果）

有谁知道为什么服务器返回404没找到？
注意：关键字只能是"ali, veli, kadir"等字符串 .