COM1端口的问题?

  电脑连接着一台label打印机,有2个程序需要打印程序,打印的数据由扫描枪扫入,在使用其中一个程序打印时,必须关掉另外一个程序(有时需要2个程序交替打印),否则就会提示"COM1端口访问被拒绝",代码如下:        public static SerialPort serialPort1 = null;
        public static string PrintLine2(string as_str, string s_Port_)
        {
            string Status = "false";
            string s_Port = s_Port_;
            switch (s_Port.ToUpper())
            {
                case "COM1":
                case "COM3":
                case "COM4":
                case "COM5":
                    if (serialPort1 == null)
                    {
                        serialPort1 = new SerialPort(s_Port, 9600, Parity.None, 8, StopBits.One);
                    }
                    if (!serialPort1.IsOpen)
                    {
                        serialPort1.Open();
                    }
                    serialPort1.Write(as_str);
                    Status = "ok";
                    break;
            }
            return Status;
        }