[ASP.NET]GridViewからExcel出力する

Default.aspx.cs

using System;
using System.Collections.Generic;
using System.Data;
using System.IO;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using ClassLibrary1;
using WebApplication1.Services;

namespace WebApplication1
{
    public partial class _Default : System.Web.UI.Page
    {
        private PersonService _service = new PersonService();
        private List<Person> dataList = new List<Person>() {
                     new Person() { ID = 1, Name = "TEST01", Address = "Tokyo" },
                     new Person() { ID = 2, Name = "TEST02", Address = "Yokohama" },
                     new Person() { ID = 3, Name = "TEST03", Address = "Osaka" },
                     new Person() { ID = 4, Name = "TEST04", Address = "Kyoto" },
                     new Person() { ID = 5, Name = "TEST05", Address = "Kobe" }
                };


        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack) 
            {               
                DataSet ds = _service.CreateDataSet<Person>(dataList);
                PersonGridView.DataSource = ds;
                PersonGridView.DataBind();
            }
        }

        //htmlのtableタグで出力したデータをxlsファイルとして無理やり認識させている
        //このためxlsx形式だとうまくいかない
        //また、日本語が文字化けするのでエンコーディングしてあげること
        protected void btnExcelExport_Click(object sender, EventArgs e)
        {
            var path = Server.MapPath("./Report/" + "Sample.xls");
            FileInfo fi = new FileInfo(path);
            StringWriter stringWriter = new StringWriter();
            HtmlTextWriter htmlWriter = new HtmlTextWriter(stringWriter);

            DataGrid dataGrid = new DataGrid();
            DataSet ds = _service.CreateDataSet<Person>(dataList);
            dataGrid.DataSource = ds;
            dataGrid.DataBind();

            dataGrid.RenderControl(htmlWriter);
            string directory = path.Substring(0, path.LastIndexOf("\\"));
            if (!Directory.Exists(directory))
            {
                Directory.CreateDirectory(directory);
            }
            StreamWriter sw = new StreamWriter(path, true);
            stringWriter.ToString().Normalize();
            sw.Write(stringWriter.ToString());
            sw.Flush();
            sw.Close();
            WriteAttachment(fi.Name, "application/vnd.ms-excel", stringWriter.ToString());
            //WriteAttachment(fi.Name, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", stringWriter.ToString());
        }

        public static void WriteAttachment(string fileName, string fileType, string content) 
        {
            HttpResponse response = HttpContext.Current.Response;
            response.ClearHeaders();
            response.AppendHeader("Content-Disposition", "attachment;filename=" + fileName);
            response.ContentType = fileType;
            //Encodingここから
            response.ContentEncoding = System.Text.Encoding.Unicode;
            response.BinaryWrite(System.Text.Encoding.Unicode.GetPreamble());
            //ここまで
            response.Write(content);
            response.End();
        }
    }
}