C# COM拒绝访问

C#程序读取传感器的数据，传感器由串口连接。现在问题是，我如果在运行程序前就连接好了传感器，然后打开串口就报出“COM3拒绝访问”，然后我只要把传感器拔了重插一下，再打开就可以了。或者在程序运行前不要连接传感器，等程序打开后再连接传感器，然后打开串口也没问题。
可是这样很麻烦，每次运行程序都要重新拔插一次。我用其他串口工具就不需要这样，一直连接的情况下，任何时候打开串口都能正常收到数据。所以不知道我的程序问题在哪？

 private void com_data_receive(object sender, SerialDataReceivedEventArgs e)
        {
            if (Closing) return;

            try
            {
                Listening = true;
                byte[] buf = new byte[24];
                receive_count += 1;
                COM.Read(buf, 0, 24);
                 COM.DiscardInBuffer();
                
                string receive_data = "";

                for (int i = 0; i < buf.Length; i++)
                {
                    receive_data += buf[i].ToString("X2");
                }
                string temperature = get_temperature(receive_data.Substring(36, 4)).ToString("0.00");
                string humility = get_humility(receive_data.Substring(40, 4)).ToString("0.00");
                string illumination = get_illumination(receive_data.Substring(44, 4)).ToString("0.00");

                this.Invoke((EventHandler)(delegate
                {
                    ListViewItem lvi = new ListViewItem(receive_count.ToString());
                    lvi.SubItems.Add(temperature);
                    lvi.SubItems.Add(humility);
                    lvi.SubItems.Add(illumination);
                    listView1.Items.Add(lvi);
                }));

        }

        private void button4_Click(object sender, EventArgs e)
        {
            if (comboBox1.SelectedItem != null && comboBox2.SelectedItem != null && comboBox3.SelectedItem != null && comboBox4.SelectedItem != null && comboBox5.SelectedItem != null)
            {

                if (COM.IsOpen)
                {
                    Closing = true;  
                    while (Listening) Application.DoEvents();  

                    COM.Close();
                    COM.Dispose();
                    Closing = false;
                }
                else
                {
                    com_setting();
                    try
                    {
                        COM.Open();
                        COM.DataReceived += new SerialDataReceivedEventHandler(com_data_receive);
                    }
                    catch (Exception ex)
                    {
                        COM.Close();
                        COM.Dispose();
                        COM = new SerialPort();
                        MessageBox.Show(ex.Message);
                    }

                }
                button4.Text = COM.IsOpen ? "关闭" : "开始";
            }
            else
            {
                MessageBox.Show("请设置参数");
            }
        }
              
        private void com_setting()
        {
              COM.PortName = comboBox1.Text;
              COM.NewLine = "\r\n";
              COM.RtsEnable = true;
              COM.BaudRate = int.Parse(comboBox2.Text);
              COM.DataBits = int.Parse(comboBox3.Text);
              switch (comboBox4.SelectedIndex)
              {
                  case 0: COM.Parity = Parity.None; break;
                  case 1: COM.Parity = Parity.Odd; break;
                  case 2: COM.Parity = Parity.Even; break;
                  case 3: COM.Parity = Parity.Mark; break;
                  case 4: COM.Parity = Parity.Space; break;
                  default: COM.Parity = Parity.None; break;
              }
              switch (comboBox5.SelectedIndex)
              {
                  case 0: COM.StopBits = StopBits.One; break;
                  case 1: COM.StopBits = StopBits.OnePointFive; break;
                  case 2: COM.StopBits = StopBits.Two; break;
                  default: COM.StopBits = StopBits.One; break;
              }

             
        }