XPathDocument document = new XPathDocument("books.xml");
XPathNavigator navigator = document.CreateNavigator();

Double total = (double)navigator.Evaluate("sum(descendant::book/price)");
Console.WriteLine("Total price for all books: {0}", total.ToString());

XPathDocument document = new XPathDocument("books.xml");
XPathNavigator navigator = document.CreateNavigator();

XPathExpression query = navigator.Compile("sum(descendant::book/price)");

Double total = (double)navigator.Evaluate(query);
Console.WriteLine("Total price for all books: {0}", total.ToString());

XPathDocument document = new XPathDocument("contosoBooks.xml");
XPathNavigator navigator = document.CreateNavigator();

XmlNamespaceManager manager = new XmlNamespaceManager(navigator.NameTable);
manager.AddNamespace("bk", "http://www.contoso.com/books");

Double total = (double)navigator.Evaluate("sum(descendant::bk:book/bk:price)", manager);
Console.WriteLine("Total price for all books: {0}", total.ToString());

XPathDocument document = new XPathDocument("books.xml");
XPathNavigator navigator = document.CreateNavigator();

XPathNodeIterator nodes = navigator.Select("//book");
XPathExpression query = nodes.Current.Compile("sum(descendant::price)");

Double total = (double)navigator.Evaluate(query, nodes);
Console.WriteLine("Total price for all books: {0}", total.ToString());

//引入命名空间
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() {
        XPathDocument doc = new XPathDocument("books.xml");
        XPathNavigator nav = ((IXPathNavigable)doc).CreateNavigator();
        XPathNodeIterator iter = nav.Select("/bookstore/book");
        while (iter.MoveNext()) {
            XPathNodeIterator newIter = iter.Current.SelectDescendants(XPathNodeType.Element, false);
            while (newIter.MoveNext())
                Console.WriteLine(newIter.Current.Name + ": " + newIter.Current.Value);
        }

        Console.WriteLine("Total Cost = " + nav.Evaluate("sum(/bookstore/book/price)"));
    }

}