C# 中BeginInvoke是前台线程还是后台线程?

C# 中BeginInvoke是前台线程还是后台线程?

        private void button9_Click(object sender, EventArgs e)
        {
            BeginInvoke(new Action(() =>
            {
                Thread.Sleep(99999999);
            }));

            Thread t = new Thread(() => { Thread.Sleep(99999999); });
            t.IsBackground = true;
            t.Start();
        }

BeginInvoke会卡主线程,不知道可否设置为后台线程