为什么小弟我用Substring第一次可以截取到小弟我想要的数据第二次就不行了

为什么我用Substring第一次可以截取到我想要的数据第二次就不行了
  while (txt != null)
                {
                    txt = sr.ReadLine();
                    if (txt != null)
                    {
                        sql = " select DirSirOrder_NO from T_DirSirOrder where DirSirOrder_NO='" + txt.Substring(0, txt.IndexOf(',')) + "'";
                        dtRow = ShineBLL.GetSingle(sql);
                        if (dtRow != null)
                        {
                            MessageBox.Show("数据已重复，请先查看数据！");
                            sr.Close();
                            sr.Dispose();
                            return;
                        }
                        sql1 = "select Prd_NO from A_PrdBase where Prd_NO='" + txt.Substring(15, txt.IndexOf(',')) + "'";
                        dtRow = ShineBLL.GetSingle(sql1);
                        if (dtRow == null)
                        {
                            //MessageBox.Show("发现非法商品，请查看数据！");
                            sw.WriteLine(txt);
                            //return;
                        }
[解决办法]
用Split切割成数组就是了。
string[] arr = txt.Split(',');

sql = " select DirSirOrder_NO from T_DirSirOrder where DirSirOrder_NO='" + arr[0] + "'";
...
sql1 = "select Prd_NO from A_PrdBase where Prd_NO='" + arr[1] + "'";