请教c#串口通信问题

Question

请高手指点，现在做一个中考报名读卡的程序，用到五岳鑫公司的omr读卡机，公司提供了一个dll,调用其中的方法读卡，c#该怎么从其中读取数据呢？我是初学者，读取数据时，总是出错，请叫高手指点。我
        [DllImport("omrvfapi.dll")] //调用非托管的dll
        static extern short OMR_Initialize(string nComNo, long nBaudRate);
        [DllImport("omrvfapi.dll")]
        static extern short OMR_LoadFmtFile(string filename);
        [DllImport("omrvfapi.dll")]
        static extern void OMR_ClrFmtBuf();
        [DllImport("omrvfapi.dll")]
        static extern void OMR_Exit();
        [DllImport("omrvfapi.dll")]
        static extern void OMR_StartReadPaper(int mode);
        [DllImport("omrvfapi.dll")]
        static extern int OMR_GetReadResult();
        [DllImport("omrvfapi.dll")]
        static extern void OMR_StopReadPaper();
        [DllImport("omrvfapi.dll")]
        static extern int OMR_GetResultString(ref string buffer);
        [DllImport("omrvfapi.dll")]
        static extern int OMR_GetLastError();
        [DllImport("omrvfapi.dll")]
        static extern void OMR_GetErrorString(int errno, ref string buf);
        [DllImport("omrvfapi.dll")]
        static extern int OMR_SetShiftValue(int value);
        [DllImport("omrvfapi.dll")]
        static extern int OMR_GetShiftValue();
        [DllImport("omrvfapi.dll")]
        static extern int OMR_SetLocValue(int vlue);
        public Form1()
        {
            InitializeComponent();
        }        private void button1_Click(object sender, EventArgs e)
        {
            string ls_port=this.comboBox1.Text.Trim();            int ll_baud = Convert.ToInt32(this.textBox1.Text.Trim());            int li_result = OMR_Initialize(ls_port, ll_baud);
            if (li_result < 0)
            {
                MessageBox.Show("初始化失败", "错误", MessageBoxButtons.OK);
                return;
            }
            OMR_ClrFmtBuf();
            string ls_fmtfile = this.textBox2.Text.Trim();
            if ((OMR_LoadFmtFile(ls_fmtfile)) <= 0)
            {
                MessageBox.Show("装载格式描述文件错误!", "OMR 系统信信息错误",MessageBoxButtons.OK,MessageBoxIcon.Information);
                return;
            }
            OMR_SetShiftValue(9);
            OMR_StartReadPaper(0); //mode=0 正常走纸; mode=1 快速走纸;            while (readstatus == "reading")
            {
                int ET_CODE = OMR_GetReadResult();
                if (ET_CODE == 0)
                    continue;
                else if (ET_CODE == 1)
                {
                    string ls_result = new string(' ', 2048);
                    int length = OMR_GetResultString(ref ls_result);
                    //string result=ls_result.ToString();
                    string result1 = ls_result.Substring(1, length);
                    //***-------在以下位置添加读入结果的处理代码------***
                    textBox3.AppendText(result1);
                    //***-------在以上位置添加读入结果的处理代码------***
                    OMR_StartReadPaper(0);
                }
            }