c#与Win中的多线程.控制形式

我是c#初学者。当我使用win.forms时，我有线程问题。我的应用程序冻结了。这段代码有什么问题?我用微软的例子从msdn。下面是我的代码:

    delegate void SetTextCallback(object text);
    private void WriteString(object text)
    {
        // InvokeRequired required compares the thread ID of the 
        // calling thread to the thread ID of the creating thread. 
        // If these threads are different, it returns true. 
        if (this.textBox1.InvokeRequired)
        {
            SetTextCallback d = new SetTextCallback(WriteString);
            this.Invoke(d, new object[] { text });
        }
        else
        {
            for (int i = 0; i <= 1000; i++)
            {
                this.textBox1.Text = text.ToString();
            }
        }
    }
    private void button1_Click(object sender, EventArgs e)
    {
        Thread th_1 = new Thread(WriteString);
        Thread th_2 = new Thread(WriteString);
        Thread th_3 = new Thread(WriteString);
        Thread th_4 = new Thread(WriteString);
        th_1.Priority = ThreadPriority.Highest; // самый высокий
        th_2.Priority = ThreadPriority.BelowNormal; // выше среднего
        th_3.Priority = ThreadPriority.Normal; // средний
        th_4.Priority = ThreadPriority.Lowest; // низкий
        th_1.Start("1");
        th_2.Start("2");
        th_3.Start("3");
        th_4.Start("4");
        th_1.Join();
        th_2.Join();
        th_3.Join();
        th_4.Join();
    }

 if (this.textBox1.InvokeRequired)
    {
        SetTextCallback d = new SetTextCallback(WriteString);
        // BeginInvoke posts message to UI thread asyncronously
        this.BeginInvoke(d, new object[] { text }); 
    }
    else
    {
        this.textBox1.Text = text.ToString();
    }