How can data retrive from Excel file using C#?

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.OleDb;
using System.IO;
public partial class export : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        //Always use single xls file in xlsfilepath  
       
        string ExcelPath = " D:\\xlfile";
        if (Directory.Exists(ExcelPath))
        {
            DirectoryInfo GetExeclFiles = new DirectoryInfo(ExcelPath);
            FileInfo[] Files = GetExeclFiles.GetFiles("*.xls");
            if (Files.Length > 0)
            {
                DataTable dtnewtable = new DataTable();
                DataTable dttable = new DataTable();
                string FilePath, excelConnectionString, sqlConnectionString;
                for (int k = 0; k < Files.Length; k++)
                {
                    FilePath = Files[k].FullName.ToString();
                    excelConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + FilePath + ";Extended Properties=Excel 8.0;";
                    // Create Connection to Excel Workbook
                    using (OleDbConnection connection = new OleDbConnection(excelConnectionString))
                    {
                        if (connection.State == ConnectionState.Open)
                            connection.Close();
                        if (connection.State == ConnectionState.Closed)
                            connection.Open();

                        //  here in query you should pass sheet name here relatedproducts is sheet name
                        OleDbCommand command = new OleDbCommand("Select * FROM [RelatedProducts$]", connection);
                        // Create DbDataReader to Data Worksheet
                        using (System.Data.Common.DbDataReader dr = command.ExecuteReader())
                        {
                            dttable.Load(dr);
                           
                            DataTable ProductsTable = new DataTable();
                            DataColumn Dtcolumn = new DataColumn("Product", typeof(string));
                            DataColumn Dtcolumn2 = new DataColumn("RelatedProduct", typeof(string));
                            ProductsTable.Columns.Add(Dtcolumn);
                            ProductsTable.Columns.Add(Dtcolumn2);
                 // code for one-to-many relation 
                            DataTable dt = dttable;
                            int colCount = dt.Columns.Count;
                            for (int r = 0; r < dt.Rows.Count; r++)
                            {
                                for (int i = 0; i < colCount; i++)
                                {
                                    for (int j = 0; j < colCount; j++)
                                    {
                                        if (string.IsNullOrEmpty(dt.Rows[r][i].ToString())) { i++; if (i >= colCount) break; }
                                        if (string.IsNullOrEmpty(dt.Rows[r][j].ToString())) { j++; if (j >= colCount) continue; }
                                        if (j != i)
                                        {
                                            if (string.IsNullOrEmpty(dt.Rows[r][i].ToString()) || string.IsNullOrEmpty(dt.Rows[r][j].ToString())) continue;
                                            ProductsTable.Rows.Add(dt.Rows[r][i].ToString(), dt.Rows[r][j].ToString());
                                        }
                                    }
                                }
                            }
                          

                            ////
                            connection.Close();
                            connection.Dispose();
                            GridView1.DataSource = ProductsTable;
                            GridView1.DataBind();
                         //
                        }//end of USING dataReader
                    }//end of USING sql connection
                }//end of for loop
              
            }//end of if(Files.Length > 0)
        }
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        ExportGridView();
    }
    private void ExportGridView()
    {
        string attachment = "attachment; filename=RelatedProducts.xls";
        Response.ClearContent();
        Response.AddHeader("content-disposition", attachment);
        Response.ContentType = "application/ms-excel";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);

        // Create a form to contain the grid
        HtmlForm frm = new HtmlForm();
        GridView1.Parent.Controls.Add(frm);
        frm.Attributes["runat"] = "server";
        frm.Controls.Add(GridView1);

        frm.RenderControl(htw);
        //GridView1.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();
    }
}