class Program
    {
        private Window _Window;

        static void Main(string[] args)
        {
            Program p = new Program();
            p.WPFCommand();
        }

        public void WPFCommand()
        {
            if (Thread.CurrentThread.GetApartmentState() != ApartmentState.STA)
            {
                Thread thread = new Thread(() =>
                {
                    System.Threading.ApartmentState aptState = Thread.CurrentThread.GetApartmentState();
                    Console.WriteLine("Spawned thread apartment: {0}", aptState); // <- Still MTA

                    // !!! If _Window is the member of the class, the thread will be MTA
                    // !!! otherwise STA
                    _Window = new Window();

                    System.Windows.Threading.Dispatcher.Run();
                });

                Console.WriteLine("Thread apartment state1: {0}", thread.GetApartmentState());
                thread.SetApartmentState(ApartmentState.STA);     // <- even though set as STA
                Console.WriteLine("Thread apartment state2: {0}", thread.GetApartmentState());

                thread.IsBackground = true;
                thread.Start();
                thread.Join();
            }
        }
    }