c#winform多线程假死（不能拖动窗口，窗口未响应）问题，在线等

public delegate void treeinvoke(int i);

        public Form1()

        {

            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)

        {

            Console.WriteLine("AAA");

            System.Threading.Thread th = new System.Threading.Thread(new System.Threading.ThreadStart(this.startupdate));

            th.IsBackground = true;

            th.Start();

            Console.WriteLine("BBB");

        }



        private void startupdate()

        {

            Console.WriteLine("CCC");

            this.BeginInvoke(new treeinvoke(this.UpdateTreeView), 0);

            Console.WriteLine("DDD");



        }



        private void UpdateTreeView(int j)

        {



            try

            {

                Console.WriteLine("EEE");

                Thread.Sleep(5000);

                Console.WriteLine("FFF");

            }

            catch (Exception ex)

            {



            }



        }

winform中，为什么运行点击button1的时候，会出现假死？不是异步执行的吗？为什么非要等执行完Thread.Sleep(5000);后才可以拖动窗口？