C#

i have written code in c# for USB read and write using WINUSBNET library,after some time my device automatically  un-plugged by OS and program exit with error code.
Managed (v4.0.30319)' has exited with code 0 (0x0).

private void btnGetPort_Click(object sender, EventArgs e)
        {
            cmbPort.Items.Clear();
            foreach (string portName in SerialPort.GetPortNames())
            {
                try
                {
                    using (SerialPort sp = new SerialPort(portName, 9600))
                    {
                        if (!(sp.IsOpen)) sp.Open();
                        sp.DiscardInBuffer();
                        sp.DiscardOutBuffer();
                        sp.Write("AT\r\n");
                        System.Threading.Thread.Sleep(100);
                        string response = sp.ReadExisting();
                        if (response == "OK")
                        {
                            cmbPort.Items.Add(portName);
                        }
                    }
                }
                catch (Exception ex)
                {
                    continue;
                }
            }
        }

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            bool isHandheld = false;

            var pdv1 = isHandheld == true ? new PDVHH() : new PDV();

            txtCaseID.Text = pdv1.CaseID;
            txtPDV.Text = isHandheld == true ? pdv1.PDVString : string.Empty;
            txtPDVHH.Text = isHandheld == true ? pdv1.PDVHHString : string.Empty;

        }
    }

    class basePDV
    {
        public string CaseID { get; set; }
    }

    class PDV : basePDV
    {
        public string PDVString { get; set; }
    }

    class PDVHH : basePDV
    {
        public string PDVHHString { get; set; }
    }