为什么我不能将json字符串发送到谷歌gsm推送通知服务器？

I'm beginner in android,write simple push notification application on android studio,and server side application write with PHP,every thing is ok and application work good,but i want change PHP application to asp.net ,but now i get bad request error,i think my json string is wrong,in php json define with this line:

   public function send_notification($registatoin_ids, $message) {
    // include config
    include_once './config.php';

    // Set POST variables
    $url = 'https://android.googleapis.com/gcm/send'; 
    $fields = array(
        'registration_ids' => $registatoin_ids,
        'data' => $message,
    ); 
    $headers = array(
        'Authorization: key=' . GOOGLE_API_KEY,
        'Content-Type: application/json'
    );
    // Open connection
    $ch = curl_init(); 
    // Set the url, number of POST vars, POST data
    curl_setopt($ch, CURLOPT_URL, $url);

    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

    // Disabling SSL Certificate support temporarly
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);

    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields)); 
    // Execute post
    $result = curl_exec($ch); 
    if ($result === FALSE) {
    print("");
        die('Curl failed: ' . curl_error($ch));

    }

and in asp.net write this code:

var httpWebRequest = (HttpWebRequest)WebRequest.Create("https://android.googleapis.com/gcm/send");
            httpWebRequest.ContentType = "application/json";
            //httpWebRequest.ContentType = "application/x-www-form-urlencoded;charset=UTF-8";
            httpWebRequest.Method = "POST";
            httpWebRequest.Headers.Add(string.Format("Authorization: key={0}", applicationID));
            httpWebRequest.Headers.Add(string.Format("Sender: key={0}", SENDER_ID));

string json = "{\"registration_ids\":[\"" + regId + "\"]," + "\"data\": { \"message\" : \"1234\", \"name\": \"Arvind Sharma\"}}";
                Console.WriteLine(json);
                streamWriter.Write(json);
                streamWriter.Flush();
                streamWriter.Close();

                var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();

but now i get null result,in android code write this line:

private final BroadcastReceiver mHandleMessageReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {


            String newMessage = intent.getExtras().getString(message);

what happen?why i get null value?thanks for helping.