vs2013 C# serial data received event handler

public partial class Form1 : Form
    {

        string rxdata;
        public Form1()
        {
            InitializeComponent();
            ///////////////////////Serial port Setting//////////////////////////////////
            SerialPort SerialPort1 = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);
            SerialPort1.Handshake = Handshake.None;
            SerialPort1.ReadTimeout = 500;
            SerialPort1.WriteTimeout = 500;
            ///////////////////////////-Receiving event handler-/////////////////////////////
            SerialPort1.DataReceived += new SerialDataReceivedEventHandler(SerialPort1_DataReceived);
            //////////////////////////////////////////////////////////////////////////////
        }
        private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            if (SerialPort1.IsOpen)
            {
                SerialPort1.Close();
            }
        }
        private void SerialPort1_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            MessageBox.Show("Data Received");
            SerialPort1 = (SerialPort)sender;
            rxdata = SerialPort1.ReadExisting();
            this.Invoke(new EventHandler(displaytext));
        }
        private void displaytext(object sender, EventArgs e)
        {
         //   textBox1.AppendText(rxdata); 
            richTextBox1.AppendText(rxdata);
        }