Rich Text Box Problem

namespace NetworkTool.Ping
{
  public partial class Network_Tool_Form : Form
  {
    public Network_Tool_Form()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {}

     string iP = "192.168.1.1";


    private void pingButton_Click(object sender, EventArgs e)
    {
      pingsSent = 0;
      pingResultRichTextBox.Clear();
      pingResultrichTextBox.Text += "Pinging " + iP + " with 32 bytes of data:\r\n\r\n";
      
       //HERE THE MESSAGE DISPLAYED ON RICHTEXTBOX CORRECTLY.  :-\ 
      SendPing();
      //IT GIVES ME "", ALTHOUGH IT WAS THE SAME PREVIOUS MESSAGE. :confused: 
      pingResultrichTextBox.Text += messageDisplay;
    }

     private int pingsSent;
     AutoResetEvent resetEvent = new AutoResetEvent(false);
     private string messageDisplay = "";

        private void SendPing()
        {
            Ping pingSender = new Ping();              
            pingSender.PingCompleted += new PingCompletedEventHandler (pingSender_Complete);
            byte[] packetData = Encoding.ASCII.GetBytes("................................");
            PingOptions packetOptions = new PingOptions(64, true);
            pingSender.SendAsync(iP, 12000, packetData, packetOptions, pingObject.resetEvent);
        }

        private void pingSender_Complete(object sender, PingCompletedEventArgs e)
        {
          if (e.Error != null)
          {
            // AT THIS point MY RICHTEXTBOX DISPLAY THE MESSAGE.  :-\ 
            pingResultrichTextBox.Text += "An error occured: ";

            //BUT IF I PASSED THE MESSAGE TO THE STRING VARIABLE AND TRY TO DISPLAY THE STRING VARIABLE IT DOSEN"T GOT THE MESSAGE.  :doh:              
            messageDisplay = "An error occured: ";
            ((AutoResetEvent)e.UserState).Set();

          }
        }
  }
}