DataSet序列化输出到XML(并指定输出XML标准)

 public partial class Form1 : Form
    {
        DataSet ds = new DataSet();
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {        
            string str = ds.GetXml();
            SaveToXml(str);
        }
        /// <summary>
        /// 输出指定文件到指定文件
        /// </summary>
        /// <param name="str"></param>
        private void SaveToXml(string str)
        { 
            string path = "d:\\Customers.xml";
            FileInfo fi = new FileInfo(path);
            if (!fi.Exists)
            {              
                using (StreamWriter sw = fi.CreateText())
                {