Index was outside the bounds of the array.

 StreamReader sr = new StreamReader(inputFile); 
            string[] allData = sr.ReadLine();             
                foreach (string line in allData)
                {
                   
                    string[] lineItems = line.Split(',');
                    if (lineItems[1] == "t2") // here i am getting the whole line into an array and then i can check whatever line i want with the [] and return appropriate result.
                        sb.AppendLine("2," + paramPath1);
                    else
                        sb.AppendLine("a," + paramPath1);
                    

                }
                using (StreamWriter outfile =
                           new StreamWriter(outputFileName))
                {
                    outfile.Write(sb.ToString());
                    outfile.Close();

                }