public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)

        {

            CheckForIllegalCrossThreadCalls = false;

            this.label1.Text = "aaaaa";

            Thread tr = new Thread(testc);

            tr.Start();

        }

        private void testc()

        {

            for (int i = ; i < ; i++)

            {

                Thread.Sleep();

                MessageBox.Show("a");

            }

        }

    }