程序报argumentoutofrangexception

void comm_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            
              int n = comm.BytesToRead;           
              byte[] buf = new byte[n];
            
            comm.Read(buf, 0, n);//读取缓冲数据

            this.Invoke((EventHandler)(delegate
            {
                //直接按ASCII规则转换成字符串
                builder.Append(Encoding.ASCII.GetString(buf, 0, n));
                string str_temp = builder.ToString();
                string lonStr = null;
                string latStr = null;
                string sulvStr = null;
                string hangxiangStr = null;
                string strGPS = null;
                int indexOfRMC = str_temp.LastIndexOf("$GPRMC");
                strGPS = str_temp.Remove(0, indexOfRMC);
                if (indexOfRMC != -1 && strGPS.Length > 10)
                {                                        
                    lonStr = getStrIndex(3, strGPS);//运行到这里报错argumentoutofrangexception，注释掉这个if语句就没事了
                    latStr = getStrIndex(5, strGPS);
                    hangxiangStr = getStrIndex(8, strGPS);
                    sulvStr = getStrIndex(7, strGPS);
                     builder.Remove(0, builder.Length);
                }
                textGet.Text += strGPS;
             }));
           
        }

private String getStrIndex(int index, String str)
        {
            
            String result = null;
            char[] str_temp = new char[str.Length];
            str_temp = str.ToCharArray();
            int sumDot = 0;
            int fistDot = 0;
            //获得总的逗号数
            for (int i = 0; i < str.Length; i++)
            { 
                if (str_temp[i] == ',')
                {
                    sumDot++; 
                }
                //取得第一个逗号所在的序号
                if (sumDot == 1)
                {
                    fistDot = i;
                }
            }
            //边界判断，如果给定的逗号序号小于0或者大于逗号总数则退出
            if (index < 0 || index > sumDot)
            {
                result = "0";
            }
            else
            {
                //分情况取得相应值
                if (index == 0)
                {
                    result = str.Substring(0, fistDot);
                }
                else if (index == sumDot)
                {
                    int endDot = 0;
                    for (int i = 0; i < str.Length; i++)
                    {
                        int dotTemp = 0;
                        if (str_temp[i] == ',')
                        {
                            dotTemp++;
                        }
                        if (sumDot == dotTemp)
                        {
                            endDot = i;
                        }
                    }
                    result = str.Substring(endDot + 1, str.Length - endDot);
                }
                else
                {
                    int p1 = -1; int p2 = -1;
                    int dotNum = 0;
                    Char p;
                    for (int i = 0; i < str.Length; i++)
                    {
                        p = str_temp[i];
                        if (p == ',')
                        {
                            dotNum++;
                            if (dotNum < index)
                            {
                                continue;
                            }
                            if (p1 == -1)
                            {
                                p1 = i + 1;
                            }
                            else
                            {
                                p2 = i;
                                break;
                            }
                        }
                    }
                    result = str.Substring(p1, p2 - p1);
                }
            }
            return result;
        }

补充：.NET技术 ,  C#