How to export datagridview contents to excel

This Datagrid to Excel:

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.IO;

public partial class ExcelSheet_ExportExcel : System.Web.UI.Page
{
    GridView grdStudentMarks = new GridView();
    Button btnExportToGrid = new Button();

    protected void Page_Load(object sender, EventArgs e)
    {
       
        // SqlDataSource sds = new SqlDataSource(@"Data Source=Master\SQLEXPRESS; Initial Catalog=shan; User Id=shan; Password=Raju", "select * from MarkSheet");
       
       
        grdStudentMarks.DataSource = sds;
        grdStudentMarks.DataBind();
       // Form.Controls.Add(grdStudentMarks);
        Form.Controls.Add(btnExportToGrid);
       
        string path = Context.Server.MapPath("Download.aspx");
        btnExportToGrid.Text = "Export To Grid ";
        btnExportToGrid.Click += new EventHandler(this.btnExportToGrid_Click);
    }
    void btnExportToGrid_Click(object sender, EventArgs e)
    {
        ExportGridToExcel(grdStudentMarks, "DynamicsDunction");
    }
    private void ExportGridToExcel(GridView grdGridView, string fileName)
    {
        IList list = new ArrayList();

        Response.Clear();

        Response.AddHeader("content-disposition", string.Format("attachment;filename={0}.xls", fileName));
        Response.Charset = "";

        Response.ContentType = "application/vnd.xls";

        StringWriter stringWrite = new StringWriter();
        HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);

        grdGridView.RenderControl(htmlWrite);

        Response.Write(stringWrite.ToString());
        Response.Flush();
        Response.End();

    }
    public override void VerifyRenderingInServerForm(Control control)
    {

    }
}