远程服务器返回了一个错误:NotFound

我有一个问题，在WindowsPhone上打开WebResponse，总是当我打开WebResponse得到一个异常。我已经尝试了许多不同的方法，但当我使用异步方法总是得到一个异常。请帮帮我。

异常:

System.Net类型的异常。在System.Net.ni.DLL中发生了"web异常"，但未在用户代码中处理附加信息:远程服务器返回了一个错误:NotFound.

private async void btLogin_Click(object sender, RoutedEventArgs e) {
        Uri uri = new Uri("http://ip-address/users/User/jan_kowalski/a");
        HttpWebRequest webRequest = WebRequest.Create(uri) as HttpWebRequest;
        webRequest.Method = "POST";
        webRequest.ContentType = "application/json";
        try {
            using (Stream ss = await webRequest.GetRequestStreamAsync()) {
                await ss.WriteAsync(new byte[0], 0, 0);
                await ss.FlushAsync();
            }
            //using (WebResponse webResponse = await webRequest.GetResponseAsync()) {       // <-- Here there is an exception or...
            //    using (StreamReader reader = new StreamReader(webResponse.GetResponseStream())) {
            //        string data = await reader.ReadToEndAsync();
            //        Log.d(TAG, data);
            //    }
            //}
            webRequest.BeginGetResponse(async (x) => {
                WebResponse webResponse = webRequest.EndGetResponse(x);         // <-- ...or here there is an exception
                StreamReader reader = new StreamReader(webResponse.GetResponseStream());
                string text = await reader.ReadToEndAsync();
                Log.d(TAG, text);
            }, null);
        } catch (WebException exc) {
            Log.e(TAG, exc.Message + "   'n" +
                exc.Source + "'n status:" +
                exc.Status + "'n" +
                exc.Response + "   'n " +
                exc.StackTrace);
        } catch (Exception ex) {
            Log.e(TAG, ex.Message + "'n stackTrace: " + ex.StackTrace);
        }
}

我注意到Android和IOS可以访问更多的同步方法。当他们使用时，一切正常，我从服务器得到正确的答案。但是同步方法在Windows phone上是不可用的!

            Uri uri = new Uri("http://ip-address/users/User/jan_kowalski/a");
            WebRequest webRequest = WebRequest.Create(uri);
            webRequest.Method = "POST";
            webRequest.ContentType = "application/json";
            try {
                using (Stream stream = webRequest.GetRequestStream()) {
                    stream.Write(new byte[0], 0, 0);
                }
                using (WebResponse stream = webRequest.GetResponse()) {
                    using (StreamReader reader = new StreamReader(stream.GetResponseStream())) {
                        string data = reader.ReadToEnd();
                        Console.WriteLine(data);
                    }
                }
            }catch(Exception ex) {
                Log.e(TAG, null, ex);
            }

var client = new HttpClient();
var res = await client.PostAsync("http://ip-address/users/User/jan_kowalski/a", new StringContent("hello, world"));
if (res.IsSuccessStatusCode)
{
    var response = await res.Content.ReadAsStringAsync();
}