public partial class MainWindow : Window
{
 ......
    private RX consumer = new RX();

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        try
        {
            Task backgroundDBTask = Task.Factory.StartNew(() =>  { Consumer(consumer);}, TaskCreationOptions.LongRunning);
        }
    }

   public void Consumer(Consumer consumer)
    {

        while (true)
        {
            Thread.Sleep(1000);
             .......
            Dispatcher.BeginInvoke(new Action(() =>
            {
                mylbl.Content = value.ToString();
            }), DispatcherPriority.Background);
        }
    }

public partial class MainWindow : Window
{
 ....
    private RX consumer = new RX();

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        try
        {
            Task backgroundDBTask = Task.Factory.StartNew(() => { consumer.ProcessMessages(); }, TaskCreationOptions.LongRunning);
        }
    }

 }


public class RX
{

   public void ProcessMessages()
    {

        while (true)
        {
            Thread.Sleep(1000);

            ....

            var m_dispatcher = Application.Current.MainWindow;

            m_dispatcher.Dispatcher.BeginInvoke(new Action(() =>
            {
                mylbl.Content = value.ToString();
            }), DispatcherPriority.Background);

        }
    }

}