kjun.kr (kjcoder.tistory.com)

        private void SendNotification()
        {
            try
            {
                string url = @"https://fcm.googleapis.com/fcm/send";
                WebRequest tRequest = WebRequest.Create(url);
                tRequest.Method = "post";
                tRequest.ContentType = "application/json";

                // 디바이스 하나
                //string deviceId = "device1-token";
                //var data = new
                //{
                //    to = deviceId,
                //    notification = new
                //    {
                //        body = "This is the message",
                //        title = "This is the title"
                //    }
                //};

                // 디바이스 여러개
                List<string> diviceList = new List<string>() {
                    "device1-token", "device2-token"
                };

                var data = new
                {
                    registration_ids = diviceList,
                    notification = new
                    {
                        body = "This is the message",
                        title = "This is the title"
                    }
                };

                string jsonss = Newtonsoft.Json.JsonConvert.SerializeObject(data);
                Byte[] byteArray = Encoding.UTF8.GetBytes(jsonss);
                tRequest.Headers.Add(string.Format("Authorization: key=", ServerKey]));
                tRequest.Headers.Add(string.Format("Sender: id=", SenderID));
                tRequest.ContentLength = byteArray.Length;
                tRequest.ContentType = "application/json";
                using (Stream dataStream = tRequest.GetRequestStream())
                {
                    dataStream.Write(byteArray, 0, byteArray.Length);

                    using (WebResponse tResponse = tRequest.GetResponse())
                    {
                        using (Stream dataStreamResponse = tResponse.GetResponseStream())
                        {
                            using (StreamReader tReader = new StreamReader(dataStreamResponse))
                            {
                                String sResponseFromServer = tReader.ReadToEnd();
                                Console.Write(sResponseFromServer);
                            }
                        }
                    }
                }
            }