Call delegate in the main thread

Question

Let's say there is a library that handles events asynchronously, e.g. UDP broadcasting. I would like to be able to pass a delegate to this library and make sure that delegate is executed in the thread where it was defined.

public void Run(Action<string> action)
{
  var PORT = 10000;
  var udpClient = new UdpClient();

  // Start listener

  udpClient.Client.Bind(new IPEndPoint(IPAddress.Any, PORT));

  var endpoint = new IPEndPoint(0, 0);
  //var scheduler = TaskScheduler.Current; // No exception but executed in another thread
  var scheduler = TaskScheduler.FromCurrentSynchronizationContext(); // Raises exception

  Task.Factory.StartNew(() =>
  {
    while (true)
    {
      action(Encoding.UTF8.GetString(udpClient.Receive(ref endpoint)));
    }
  }, CancellationToken.None, TaskCreationOptions.None, scheduler); 

  // Start sender

  var data = Encoding.UTF8.GetBytes("PING");

  udpClient.Send(data, data.Length, "255.255.255.255", PORT);

  var aTimer = new System.Timers.Timer();

  aTimer.Elapsed += new ElapsedEventHandler(OnTimer);
  aTimer.Interval = 1000;
  aTimer.Enabled = true;

  void OnTimer(object source, ElapsedEventArgs e)
  {
    var data = Encoding.UTF8.GetBytes("PONG");
    udpClient.Send(data, data.Length, "255.255.255.255", PORT);
  }
}

Then I check in which thread my delegate was executed.

Console.WriteLine("Start on Thread : " + Thread.CurrentThread.ManagedThreadId);
        
new Beacon().Run(response =>
{
  Console.WriteLine("Continue on Thread : " + Thread.CurrentThread.ManagedThreadId);
});

If I use TaskScheduler.Current nothing changes, the delegate is executed in another thread. If I use TaskScheduler.FromCurrentSynchronizationContext, I get an exception below.

The current SynchronizationContext may not be used as a TaskScheduler

Questions

• Why I can't use synchronization context in .NET core or how to use it properly?
• Is there a better way of marshaling delegate call to the main thread?
• Are BackgroundWorker and EventLoopScheduler obsolete now?