Export Gridview to Excel in asp.net

//In Button Click


HtmlForm form = new HtmlForm();
        string attachment = "attachment; filename=Report.xls";
        Response.ClearContent();
        Response.AddHeader("content-disposition", attachment);
        Response.ContentType = "application/ms-excel";
        StringWriter stw = new StringWriter();
        HtmlTextWriter htextw = new HtmlTextWriter(stw);
        form.Controls.Add(GridView1);
        this.Controls.Add(form);
        form.RenderControl(htextw);
        Response.Write(stw.ToString());
        Response.End();