In one of my current project, I had to develop and integrate the Android Push notification functionality in my server side C# code.For this purpose, i used the Google Cloud Messaging service.

Here is code for the same.

 public void SendNotification(string deviceId, string message, int badgeCount)                {
                    //API Key created in Google project 
                    string GoogleAppID = XXXX-------XX;//Google App Id

                    //Project ID created in Google project
                    var SENDER_ID = XXXX-------XX;//Google App Id

                    //Registration Id created by Android App i.e. DeviceId
                    string regID = deviceId.Trim();

                    WebRequest webRequest;
                    webRequest = WebRequest.Create("https://android.googleapis.com/gcm/send");
                    webRequest.Method = "post";
                    webRequest.ContentType = " application/x-www-form-urlencoded;charset=UTF-8";
                    webRequest.Headers.Add(string.Format("Authorization: key={0}", GoogleAppID));
                    webRequest.Headers.Add(string.Format("Sender: id={0}", SENDER_ID));

                    string postData = string.Format("collapse_key=score_update&time_to_live=108&delay_while_idle=1&data.message={0} &data.time={1} &data.badge={3} &data.sound={4}&registration_id={2}", message, DateTime.UtcNow, regID, badgeCount, "default");

                    Byte[] byteArray = Encoding.ASCII.GetBytes(postData);

                    webRequest.ContentLength = byteArray.Length;

                    Stream dataStream = webRequest.GetRequestStream();
                    dataStream.Write(byteArray, 0, byteArray.Length);
                    dataStream.Close();

                    WebResponse webResponse = webRequest.GetResponse();

                    dataStream = webResponse.GetResponseStream();

                    using (StreamReader streamReader = new StreamReader(dataStream))
                    {
                        String sResponseFromServer = streamReader.ReadToEnd();
                        streamReader.Close();
                        dataStream.Close();
                        webResponse.Close();
                    }
                }

In above code, few things are prerequisite which are as follows.

deviceId.
message.
badgeCount.
GoogleAppID.
SENDERID .

DeviceId is needed to identify which device the notification has to be sent to.

Message contains the notification text.

badgeCount is useful for showing badgecount on the app icon in IOS device.

GoogleAppID is the App Id for the application to be picked from google project.

SENDERID is the same as GoogleAppID.

Notifications are sent through "https://android.googleapis.com/gcm/send" web request.