asp.net SQL数据库导入到XML

SqlConnection conn = new SqlConnection("server=.;database=DbXml;uid=sa;pwd=sa");

protected void btnXML_Click(object sender, EventArgs e)
    {
        
       string xml = "select title,content,creattime from tb_test";
        conn.Open();
        SqlCommand cmd = new SqlCommand(xml,conn);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
       ds.WriteXml(Server.MapPath("XML3.xml"));
        conn.Close();
        ClientScript.RegisterStartupScript(this.GetType(), "", "<script>alert('导入成功')</script>");

  }