problem in Exporting data from grid view to excel file in asp.net

 protected void btnExport_Click(object sender, EventArgs e)
    {

        DataTable dt = new DataTable();
        dt = Session["data"] as DataTable;

        if (dt == null)
        {
            throw new Exception("No Records to Export");
        }
        string Path = "D:\\ImportExcelFromDatabase\\Karvyexcel_" + DateTime.Now.Day.ToString() + "_" + DateTime.Now.Month.ToString() + ".xls";

        //string Path = "D:\\Sujatha\\Karvy_27Nov2012\\Inventory_" + DateTime.Now.Day.ToString() + "_" + DateTime.Now.Month.ToString() + ".xls";
        FileInfo FI = new FileInfo(Path);
        StringWriter stringWriter = new StringWriter();
        HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWriter);
        DataGrid DataGrd = new DataGrid();
        DataGrd.DataSource = dt;
        DataGrd.DataBind();

        DataGrd.RenderControl(htmlWrite);
        ////string directory = Path.Substring(0, Path.LastIndexOf("\\"));// GetDirectory(Path);
        ////if (!Directory.Exists(directory))
        ////{
        ////    Directory.CreateDirectory(directory);
        ////}

        //System.IO.StreamWriter vw = new System.IO.StreamWriter(Path, true);
        stringWriter.ToString().Normalize();
        //vw.Write(stringWriter.ToString());
        //vw.Flush();
        //vw.Close();
        WriteAttachment(FI.Name, "application/vnd.ms-excel", stringWriter.ToString());

       
    }
    public static void WriteAttachment(string FileName, string FileType, string content)
    {
        HttpResponse Response = System.Web.HttpContext.Current.Response;
        Response.ClearHeaders();
        Response.AppendHeader("Content-Disposition", "attachment; filename=" + FileName);
        Response.ContentType = FileType;
        Response.Write(content);
        Response.End();

    }

private void BindCustomerData()
    {
        try
        {
            string strSelect = "select * from CustomerData";
            DataSet ds = new DataSet();
            cnn.ConnectionString = connStr;
            // if the connection will be closed the below code poen the connection when the page will be loaded.
            if (cnn.State == ConnectionState.Closed)
            {
                cnn.Open();
            }
            // here i am using the store procedure named tb_gallery_insert to insert the record inside the database.
            SqlDataAdapter da = new SqlDataAdapter(strSelect, cnn);
            da.Fill(ds);
            if (ds.Tables[0].Rows.Count > 0)
            {
                gvData.DataSource = ds.Tables[0];
                gvData.DataBind();

                Session["data"] = ds.Tables[0];

            }
            else
            {
                lblMessage.Text = "No Records Found";
                lblMessage.ForeColor = System.Drawing.Color.Red;
            }
        }
        catch (Exception ex)
        {
            lblMessage.Text = ex.Message;
            lblMessage.ForeColor = System.Drawing.Color.Red;
        }      
        
    }