System Xml C# by API

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Xml;
using System.Xml.XPath;
public class MainClass {
    public static void Main() {
        XmlDocument doc = new XmlDocument();
        XmlReaderSettings rs = new XmlReaderSettings();
        rs.Schemas.Add(null, "books.xsd");
        rs.ValidationType = ValidationType.Schema;
        XmlReader rdr = XmlReader.Create("books.xml", rs);
        doc.Load(rdr);
        XPathNavigator nav = doc.CreateNavigator();
        if (nav.CanEdit) {
            XPathNodeIterator iter = nav.Select("/bookstore/book/price");
            while (iter.MoveNext()) {
                iter.Current.SetTypedValue("Invalid");
            }
        }
        doc.Save("newbooks.xml");
    }
}