Read excel file and add new colunm in excel using asp.net ,C#

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Text;

using System.Windows.Forms;

using System.Windows;

using System.Xml;

using System.Collections;

using System.IO;

using System.Globalization;

using System.Data;

using System.Data.OleDb;

using System.Reflection;

using System.Resources;

using System.Globalization;

using Microsoft.Office.Interop;

using Excel = Microsoft.Office.Interop.Excel;

private void button1_Click(object sender, EventArgs e)

        {

            saveexl();

        }

public void saveexl()

        {

            string strExcelFilePath = @"D:\\fdlog.xlsx";

            DataTable dt = new DataTable();

            string conStr = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + strExcelFilePath + ";Extended Properties=Excel 8.0";

            OleDbConnection connExcel = new OleDbConnection(conStr);

            OleDbCommand cmdExcel = new OleDbCommand();

            OleDbDataAdapter oda = new OleDbDataAdapter();

            cmdExcel.Connection = connExcel;

            //Get the name of First Sheet

            connExcel.Open();

            DataTable dtExcelSchema;

            dtExcelSchema = connExcel.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);

            string SheetName = dtExcelSchema.Rows[0]["TABLE_NAME"].ToString();

            connExcel.Close();

            //Read Data from First Sheet

            connExcel.Open();

            cmdExcel.CommandText = "SELECT * From [" + SheetName + "]";

            oda.SelectCommand = cmdExcel;

            oda.Fill(dt);

            connExcel.Close();

            string temp = "";

            string filePath = @"D:\\sss.csv";

            string delimiter = "~";

            StringBuilder sb = new StringBuilder();

            DateTime dat = DateTime.Now;

            Microsoft.Office.Interop.Excel.Worksheet sheet = new Microsoft.Office.Interop.Excel.Worksheet();

            object misValue = System.Reflection.Missing.Value;

            string workbookPath = "D:/fdlog.xlsx";

            //Excel.Workbook excelWorkbook = excelApp.Workbooks.Add(misValue);

            Microsoft.Office.Interop.Excel.Workbook excelWorkbook = excelApp.Workbooks.Open(workbookPath, 0, false, 5, "", "", false, Excel.XlPlatform.xlWindows, "", true, false, 0, true, false, false);

            Microsoft.Office.Interop.Excel.Sheets excelSheets = excelWorkbook.Worksheets;

            string currentSheet = "fdlog";

                    Microsoft.Office.Interop.Excel.Worksheet excelWorksheet = (Microsoft.Office.Interop.Excel.Worksheet)excelSheets.get_Item(currentSheet);

            for (int intRowIndex = 1; intRowIndex < dt.Rows.Count; intRowIndex++)

            {

                try

                {

                    string text = dt.Rows[intRowIndex][1].ToString();

                    string code = dt.Rows[intRowIndex][0].ToString();

                                        

                    excelWorksheet.Cells[intRowIndex, 4] = text;

               

                }

                catch (Exception ex)

                {

                  

                }

            }

            string filename = "fdlog-sunil";

            excelWorkbook.SaveAs(filename + ".xlsx", misValue, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlNoChange, misValue, misValue, misValue, misValue, misValue);

            excelWorkbook.Close(true, misValue, misValue);

            excelApp.Quit();

            releaseObject(excelWorksheet);

            releaseObject(excelWorkbook);

            releaseObject(excelApp);

            System.IO.File.WriteAllText(filePath, sb.ToString());

          

        }

  private void releaseObject(object obj)
        {
            try
            {
                System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
                obj = null;
            }
            catch (Exception ex)
            {
                obj = null;
                MessageBox.Show("Exception Occured while releasing object " + ex.ToString());
            }
            finally
            {
                GC.Collect();
            }
        }