notify.cs

// Set up an intent so that tapping the notifications returns to this app:
Intent intent = new Intent (this, typeof(MainActivity));

// Create a PendingIntent; we're only using one PendingIntent (ID = 0):
const int pendingIntentId = 0;
PendingIntent pendingIntent = 
    PendingIntent.GetActivity (this, pendingIntentId, intent, PendingIntentFlags.OneShot);

// Instantiate the builder and set notification elements, including pending intent:
Notification.Builder builder = new Notification.Builder(this)
    .SetContentIntent (pendingIntent)
    .SetContentTitle ("Sample Notification")
    .SetContentText ("Hello World! This is my first action notification!")
    .SetSmallIcon (Resource.Drawable.ic_notification);

// Build the notification:
Notification notification = builder.Build();

// Get the notification manager:
NotificationManager notificationManager =
    GetSystemService (Context.NotificationService) as NotificationManager;

// Publish the notification:
const int notificationId = 0;
notificationManager.Notify (notificationId, notification);