c# serial textbox problem

Code C# - [expand]
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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 ser1
{
    public partial class Form1 : Form
    {
        private string RxString;
 
        public Form1()
        {
            InitializeComponent();
        }
 
        private void buttonStart_Click(object sender, EventArgs e)
        {
            serialPort1.PortName = "COM4";
            serialPort1.BaudRate = 9600;
 
            serialPort1.Open();
            if (serialPort1.IsOpen)
            {
                buttonStart.Enabled = false;
                buttonStop.Enabled = true;
                textBox1.ReadOnly = false;
            }
 
        }
 
        private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            // If the port is closed, don't try to send a character.
            if (!serialPort1.IsOpen) return;
 
            // If the port is Open, declare a char[] array with one element.
 
            char[] buff = new char[1];
 
            // Load element 0 with the key character.
            buff[0] = e.KeyChar;
 
            // Send the one character buffer.
            serialPort1.Write(buff, 0, 1);
 
            // Set the KeyPress event as handled so the character won't
            // display locally. If you want it to display, omit the next line.
 
            e.Handled = true;
 
        }
        private void DisplayText(object sender, EventArgs e)
        {
            textBox1.Text=RxString.Trim();
            textBox1.Show="h";
        }
 
 
       
 
        private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            RxString = serialPort1.ReadExisting();
            this.Invoke(new EventHandler(DisplayText));
 
        }
 
        private void buttonStop_Click(object sender, EventArgs e)
        {
            if (serialPort1.IsOpen)
            {
                serialPort1.Close();
                buttonStart.Enabled = true;
                buttonStop.Enabled = false;
                textBox1.ReadOnly = true;
            }
 
        }
  
 
 
        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (serialPort1.IsOpen) serialPort1.Close();
 
        }
 
        private void textBox1_TextChanged(object sender, EventArgs e)
        {
 
        }
    }
}