android-gcm | GCM Sample Client and Server | Microservice library

public override void OnNewToken(string token)
        {
            Log.Debug(TAG, "FCM token: " + token);
            SendRegistrationToServer(token);
        }

public override void OnNewToken(string token)
        {
            Log.Debug(TAG, "FCM token: " + token);
            LocalDb.InsertDeviceToken(token);
        }

 var notificationUtil = new NotificationUtil();
 notificationUtil.SendRegistrationToServer(this.ApplicationContext);

using System;
using System.Collections.Generic;
using Android.Util;
using PantAppLib.source.dbaccess;
using PantAppLib.source.models;
using WindowsAzure.Messaging;

namespace PantAppAndroid.Utils
{
    public class NotificationUtil
    {
        public void SendRegistrationToServer(Android.Content.Context context)
        {
            try
            {
                // Register with Notification Hubs
                NotificationHub hub = new NotificationHub(Constants.NotificationHubName,
                                            Constants.ListenConnectionString, context);
                var userProfile = LocalDb.GetUserProfile();
                if (userProfile != null)
                {
                    var tags = new List() { userProfile.Id };
                    var token = LocalDb.GetDeviceToken().TokenValue;
                    Registration registration = hub.Register(token, tags.ToArray());
                    var regID = registration.RegistrationId;

                    Log.Debug("MyFirebaseMsgService", $"Successful registration of ID {regID}");

                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }
    }
}

{Android.OS.NetworkOnMainThreadException: Exception of type 'Android.OS.NetworkOnMainThreadException' was thrown.
  at Java.Interop.JniEnvironment+InstanceMethods.CallObjectMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x0006e] in <42748fcc36b74733af2d9940a8f3cc8e>:0 
  at Java.Interop.JniPeerMembers+JniInstanceMethods.InvokeVirtualObjectMethod (System.String encodedMember, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x0002a] in <42748fcc36b74733af2d9940a8f3cc8e>:0 
  at WindowsAzure.Messaging.NotificationHub.Register (System.String pnsHandle, System.String[] tags) [0x00043] in <7ef3a358b177460dacd73e56198bd8f2>:0 
  at PantAppAndroid.Utils.NotificationUtil.SendRegistrationToServer (Android.Content.Context context) [0x00041] in C:\Users\weiha\source\repos\App2\PantAppAndroid\Utils\NotificationUtil.cs:36 
  --- End of managed Android.OS.NetworkOnMainThreadException stack trace ---
android.os.NetworkOnMainThreadException
    at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1513)
    at java.net.Inet6AddressImpl.lookupHostByName(Inet6AddressImpl.java:117)
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:105)
    at java.net.InetAddress.getAllByName(InetAddress.java:1154)
    at com.android.okhttp.Dns$1.lookup(Dns.java:39)
    at com.android.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:175)
    at com.android.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:141)
    at com.android.okhttp.internal.http.RouteSelector.next(RouteSelector.java:83)
    at com.android.okhttp.internal.http.StreamAllocation.findConnection(StreamAllocation.java:174)
    at com.android.okhttp.internal.http.StreamAllocation.findHealthyConnection(StreamAllocation.java:126)
    at com.android.okhttp.internal.http.StreamAllocation.newStream(StreamAllocation.java:95)
    at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:281)
    at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:224)
    at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:461)
    at com.android.okhttp.internal.huc.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:127)
    at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.connect(DelegatingHttpsURLConnection.java:89)
    at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.connect(HttpsURLConnectionImpl.java:26)
    at com.microsoft.windowsazure.messaging.Connection.executeRequest(Connection.java:213)
    at com.microsoft.windowsazure.messaging.Connection.executeRequest(Connection.java:170)
    at com.microsoft.windowsazure.messaging.Connection.executeRequest(Connection.java:130)
    at com.microsoft.windowsazure.messaging.NotificationHub.refreshRegistrationInformation(NotificationHub.java:301)
    at com.microsoft.windowsazure.messaging.NotificationHub.registerInternal(NotificationHub.java:399)
    at com.microsoft.windowsazure.messaging.NotificationHub.register(NotificationHub.java:148)
    at mono.android.view.View_OnClickListenerImplementor.n_onClick(Native Method)
    at mono.android.view.View_OnClickListenerImplementor.onClick(View_OnClickListenerImplementor.java:30)
    at android.view.View.performClick(View.java:6597)
    at android.view.View.performClickInternal(View.java:6574)
    at android.view.View.access$3100(View.java:778)
    at android.view.View$PerformClick.run(View.java:25885)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
}

public static class AppConstants
{
    public static string NotificationChannelName { get; set; } = "XamarinNotifyChannel";
    public static string NotificationHubName { get; set; } = "(my azure notification hub name)";
    public static string ListenConnectionString { get; set; } = "(my default listen shared access signature from azure portal)";
    public static string DebugTag { get; set; } = "XamarinNotify";
    public static string[] SubscriptionTags { get; set; } = { "default" };
    public static string FCMTemplateBody { get; set; } = "{\"data\":{\"message\":\"$(messageParam)\"}}";
    public static string APNTemplateBody { get; set; } = "{\"aps\":{\"alert\":\"$(messageParam)\"}}";
}

[Service(Name = "(my package name).MyFirebaseMessagingService")]
[IntentFilter(new[] {  "com.google.firebase.MESSAGING_EVENT" })]
public class FirebaseService : FirebaseMessagingService
{
    public override void OnNewToken(string token)
    {
        base.OnNewToken(token);
        Console.WriteLine("NEW_TOKEN", token);
        SendRegistrationToServer(token);
    }

    void SendRegistrationToServer(string token)
    {
        NotificationHub hub = new NotificationHub(AppConstants.NotificationHubName, AppConstants.ListenConnectionString, this);
        // register device with Azure Notification Hub using the token from FCM
        Registration reg = hub.Register(token, AppConstants.SubscriptionTags);
        // subscribe to the SubscriptionTags list with a simple template.
        string pnsHandle = reg.PNSHandle;
        hub.RegisterTemplate(pnsHandle, "defaultTemplate", AppConstants.FCMTemplateBody, AppConstants.SubscriptionTags);
    }

    public override void OnMessageReceived(RemoteMessage message)
    {
        base.OnMessageReceived(message);
        string messageBody = string.Empty;
        if (message.GetNotification() != null)
        {
            messageBody = message.GetNotification().Body;
        }
        else
        {
            messageBody = message.Data.Values.First();
        }
        try
        {
            MessagingCenter.Send(messageBody, "Update");
        }
        catch (Exception e)
        { }
        SendLocalNotification(messageBody);
    }

    void SendLocalNotification(string body)
    {
        var intent = new Intent(this, typeof(MainActivity));
        intent.AddFlags(ActivityFlags.ClearTop);
        intent.PutExtra("message", body);

        //Unique request code to avoid PendingIntent collision.
        var requestCode = new Random().Next();
        var pendingIntent = PendingIntent.GetActivity(this, requestCode, intent, PendingIntentFlags.OneShot);

        var notificationBuilder = new NotificationCompat.Builder(this)
            .SetContentTitle("XamarinNotify Message")
            .SetSmallIcon(Resource.Drawable.ic_launcher)
            .SetContentText(body)
            .SetAutoCancel(true)
            .SetShowWhen(false)
            .SetContentIntent(pendingIntent);

        if (Build.VERSION.SdkInt >= BuildVersionCodes.O)
        {
            notificationBuilder.SetChannelId(AppConstants.NotificationChannelName);
        }

        var notificationManager = NotificationManager.FromContext(this);
        notificationManager.Notify(0, notificationBuilder.Build());
    }
}