private void button1_Click(object sender, EventArgs e)
        {
            List<int> lst = new List<int>();
            for (int i = 0; i < 10; i++)
            {
                Thread myThread = new Thread(() => ThreadRun.run(i, textBox1));
                myThread.Start();
                Thread.Sleep(50);
            }
        }

    public  class ThreadRun
    {
        public static void run(int num, TextBox textBox)
        {
            addtexbox(num + "\n", textBox);
        }

        public static void addtexbox(string st, TextBox tb1)
        {
            st += "\n";
            if (tb1.InvokeRequired) tb1.Invoke(
            new Action<string>((s) =>
            {
                tb1.SelectionStart = tb1.TextLength;
                tb1.SelectionLength = st.Length - 1;


                tb1.AppendText(st);
            }), st);
            else
            {
                tb1.SelectionStart = tb1.TextLength;
                tb1.SelectionLength = st.Length - 1;

                tb1.AppendText(st);
            };
        }
    }