How to export the data in the dataset to notepad in web application

protected void btn_Download_Click(object sender, EventArgs e)
{
DataSet ds = new DataSet();
ds = GetDatatable();
string filename = "MyObCustomerCard.txt";
// Exporting Data to text file
ExportDataTabletoFile(ds.Tables[0], " ", true, Server.MapPath("~/image/MyObCustomerCard.txt"));
#region download notepad or text file.
Response.ContentType = "application/octet-stream";
Response.AppendHeader("Content-Disposition", "attachment;filename=" + filename);
string aaa = Server.MapPath("~/contents/" + filename);
Response.TransmitFile(Server.MapPath("~/image/" + filename));
HttpContext.Current.ApplicationInstance.CompleteRequest();
Response.End();
#endregion
}

public void ExportDataTabletoFile(DataTable datatable, string delimited, bool exportcolumnsheader, string file)
    {
        StreamWriter str = new StreamWriter(file, false, System.Text.Encoding.Default);
        if (exportcolumnsheader)
        {
            string Columns = string.Empty;
            foreach (DataColumn column in datatable.Columns)
            {
                Columns += column.ColumnName + delimited;
            }
            str.WriteLine(Columns.Remove(Columns.Length - 1, 1));
        }
        foreach (DataRow datarow in datatable.Rows)
        {
            string row = string.Empty;
            foreach (object items in datarow.ItemArray)
            {
                row += items.ToString() + delimited;
            }
            str.WriteLine(row.Remove(row.Length - 1, 1));
        }
        str.Flush();
        str.Close();
    }