How can i have rfid data to be converted in string data using c#

How can i have rfid data to be converted in string data using c#

I have this code that can have rfid ascii data, to the text box but what i
want is before it gets to the text box it must be converted to string data
like milk, meat, soda and fruits. I'm working on an inventory system using
RFID tags, can anyone help me please.
this is my working code i use Visual Studio C#
string RxString;
public Form1()
{
InitializeComponent();
}
private void buttonStart_Click(object sender, EventArgs e)
{
serialPort1.PortName = "COM15";
serialPort1.BaudRate = 9600;
serialPort1.Open();
if (serialPort1.IsOpen)
{
buttonStart.Enabled = false;
buttonStop.Enabled = true;
textBox1.ReadOnly = false;
}
}
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_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.AppendText(RxString);
}
private void serialPort1_DataReceived
(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
RxString = serialPort1.ReadExisting();
this.Invoke(new EventHandler(DisplayText));
}
private void textBox1_TextChanged(object sender, EventArgs e)
{
}
}
}