Unauthorized exception serialport

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C#

Arduino

serial-port

I am trying to read RFID tags and display it my inventory c# application. I have accessed it through Arduino application which gives me the right output on control monitor when I pass RFID Keychain. But when I tried to access it through visual studio, it works only once and then gives me access is denied!and also stopped working on Arduino IDE saying the port is busy!

What I have tried:

public products()
    {
        InitializeComponent();
        initializeRFIDPort();
    }

//Function  to initialize the serial port

 SerialPort RFIDPort    
 public SerialPort initializeRFIDPort()
    {

        try
        {
            RFIDPort = new SerialPort("COM4",9600,Parity.None,8, StopBits.One);
          
            if (RFIDPort.IsOpen)
                RFIDPort.Close();

            if(!RFIDPort.IsOpen)
                    RFIDPort.Open();
            }
        catch (UnauthorizedAccessException ex) {MessageBox.Show( ex.Message); }
        catch (Exception)
        {
            RFIDPort = null;
        }

        return RFIDPort;
    }

I have two scan button in my class:

 private void ScanButton_Click(object sender, EventArgs e)
    {
        try
        {

            scanButtonIsClicked = true;
            if (RFIDPort.IsOpen)
            {
                RFIDPort.DataReceived += serialPort1_DataReceived;
                textBox1.Text = "";

            }
            else
                MessageBox.Show("RFID Reader is not connected!");
        }
        catch (IOException) { MessageBox.Show("Please reconnect your device "); }
        catch (System.Exception)
        {
            MessageBox.Show("Please Try Again");
        }
     }

  private void Searchbutton_Click(object sender, EventArgs e)
    {
        scansearchbtn = true;
        scanButtonIsClicked = false;

        try
        {
            if (RFIDPort.IsOpen)
            {
                 RFIDPort.DataReceived += serialPort1_DataReceived;

                textBox2.Text = "";

            }
            else { MessageBox.Show("RFID Reader is not connected!"); }
        }
        catch (IOException) { MessageBox.Show("Please reconnect your device ");}
        }

private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
    {
        try
        {

            string line = RFIDPort.ReadLine();

            if (scanButtonIsClicked == true)
                this.BeginInvoke(new LineReceivedEvent(LineReceived), line);
            else
                this.BeginInvoke(new LineReceivedEvent(Line2Received), line);
        }
        catch (Exception){ MessageBox.Show("Can't Read from RFID Device.Please try again"); }

    }

    private delegate void LineReceivedEvent(string line);

    private void LineReceived(string line)
    {
        textBox2.Text = line;

    }
    private void Line2Received(string line)
    {
        textBox1.Text = line;

    }

and then I closed serial port in Forum-Closing. when I close the program which contains RFIDPort.Close(), also an exception that I/O is aborted.
please If anyone can lead me to the right way. I have try many times!!