用filestream读取文件的时候为什么显示很多异常？


有问题的部分代码如下：
  OpenFileDialog fileDialog = new OpenFileDialog();
            fileDialog.Title = "打开文件";
            fileDialog.Filter = "Nav文件(*.**n)|*.**n";
            if (fileDialog.ShowDialog() == DialogResult.OK)
            {
                list = new List<NavFormat>();
                //打开文件
                FileStream fs = new FileStream(fileDialog.FileName, FileMode.Open, FileAccess.Read);
                StreamReader sr = new StreamReader(fs);
                //循环读取数据
                while (true)
                {
                    if (sr.ReadLine().Substring(60) == "END OF HEADER")
                    {
                        break;
                    }
                }
                string data;
                while (true)
                {
                    data = sr.ReadLine();
                    if (data == null)
                        break;
                    NavFormat objFormat = new NavFormat();
                    objFormat.thisToc = new Toc();
                    //第一行信息
                    objFormat.satPrn = data.Substring(0, 2);
                    objFormat.thisToc.year = Convert.ToInt32(data.Substring(3, 2)) + 2000;
                    objFormat.thisToc.month = Convert.ToInt32(data.Substring(6, 2));
                    objFormat.thisToc.day = Convert.ToInt32(data.Substring(9, 2));
                    objFormat.thisToc.h = Convert.ToInt32(data.Substring(13, 1));
                    objFormat.thisToc.m = Convert.ToInt32(data.Substring(16, 1));
                    objFormat.thisToc.s = ConvertToDouble(data.Substring(18, 4));
                    objFormat.a0 = ConvertToDouble(data.Substring(23, 18));
                    objFormat.a1 = ConvertToDouble(data.Substring(42, 18));
                    objFormat.a2 = ConvertToDouble(data.Substring(61, 18));
                    //第二行信息
                    data = sr.ReadLine();
                    objFormat.IODE = ConvertToDouble(data.Substring(4, 18));
                    objFormat.Crs = ConvertToDouble(data.Substring(23, 18));
                    objFormat.DeltaN = ConvertToDouble(data.Substring(42, 18));
                    objFormat.M0 = ConvertToDouble(data.Substring(61, 18));
                    //第三行信息
                    data = sr.ReadLine();
                    objFormat.Cuc = ConvertToDouble(data.Substring(4, 18));
                    objFormat.e = ConvertToDouble(data.Substring(23, 18));
                    objFormat.Cus = ConvertToDouble(data.Substring(42, 18));
                    objFormat.sqrtA = ConvertToDouble(data.Substring(61, 18));
                    //第四行信息
                    data = sr.ReadLine();
                    objFormat.toe = ConvertToDouble(data.Substring(4, 18));
                    objFormat.Cic = ConvertToDouble(data.Substring(23, 18));
                    objFormat.oumiga0 = ConvertToDouble(data.Substring(42, 18));
                    objFormat.Cis = ConvertToDouble(data.Substring(61, 18));
                    //第五行信息
                    data = sr.ReadLine();
                    objFormat.i0 = ConvertToDouble(data.Substring(4, 18));
                    objFormat.Crc = ConvertToDouble(data.Substring(23, 18));
                    objFormat.w = ConvertToDouble(data.Substring(42, 18));
                    objFormat.OumigaDot = ConvertToDouble(data.Substring(61, 18));
                    //第六行信息
                    data = sr.ReadLine();
                    objFormat.IDot = ConvertToDouble(data.Substring(4, 18));
                    objFormat.Code = ConvertToDouble(data.Substring(23, 18));
                    objFormat.GPSweek = ConvertToDouble(data.Substring(42, 18));
                    objFormat.L2_flag = ConvertToDouble(data.Substring(61, 18));
                    //都两个空行，不存储数据
                    sr.ReadLine();
                    sr.ReadLine();
                    //添加数据到list集合中
                    list.Add(objFormat);
                }
            }