Improper Usage of Invoke

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace test
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            //  Invoke does not work without the control
            //  handle first being created.  Since I am 
            //  starting the worker threads in the constructor, 
            //  I have to force creation of this handle early.
            //  Remove this code and see what happens!
            while (!IsHandleCreated)
            {
                // force handle creation
                IntPtr temp = Handle;
            }
            ThreadPool.QueueUserWorkItem(Foo);
            ThreadPool.QueueUserWorkItem(Foo);
            ThreadPool.QueueUserWorkItem(Foo);
        }
        public void Foo(object o)
        {
            while (true)
            {
                rtb.Invoke(new StringDelegate(UpdateDisplay), 
                    new object[] { "Foo!\n" });
                //Thread.Sleep(10);
            }
        }
        public void UpdateDisplay(string text)
        {
            this.rtb.Text += text;
        }

        public delegate void StringDelegate(string foo);
    }
}