多线种中为控件赋值，怎么做？小弟我的代码赋值不了

MDI父using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Windows.Forms;namespace WhyThread{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        private void Form1_Load(object sender, EventArgs e)        {            Form2 f = new Form2(this);            f.Show();        }    }}子窗口using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Windows.Forms;using System.Threading;namespace WhyThread{    public partial class Form2 : Form    {        Thread[] threads;        public Form2(Form1 parent)        {            InitializeComponent();            MdiParent = parent;        }        private void Form2_Load(object sender, EventArgs e)        {        }        void Do()        {            richTextBox1.Text += DateTime.Now.ToString();        }        void ThreadProc(Object obj)        {            (obj as Form2).Do();        }        private void button1_Click(object sender, EventArgs e)        {            threads = new Thread[100];            for (int i = 0; i < threads.Length; i++)            {                threads[i] = new Thread(ThreadProc);                threads[i].IsBackground = true;                threads[i].Start(this);            }        }    }}