在C#中，使用System.Xml.XPath命名空間可以方便地處理XPath表達式

假設我們有以下XML文檔：

<?xml version="1.0" encoding="UTF-8"?>
<books>
  <book id="1">
    <title>Book 1</title>
    <author>Author 1</author>
  </book>
  <book id="2">
    <title>Book 2</title>
    <author>Author 2</author>
  </book>
  <book id="3">
    <title>Book 3</title>
    <author>Author 3</author>
  </book>
</books>

現在，我們將使用C#和XPath表達式來查詢此XML文檔。

1. 選擇所有書籍的標題：

using System;
using System.Xml;
using System.Xml.XPath;

class XPathExample
{
    static void Main()
    {
        string xml = @"
          <books>
            <book id='1'>
              <title>Book 1</title>
              <author>Author 1</author>
            </book>
            <book id='2'>
              <title>Book 2</title>
              <author>Author 2</author>
            </book>
            <book id='3'>
              <title>Book 3</title>
              <author>Author 3</author>
            </book>
          </books>";

        XPathDocument doc = new XPathDocument(xml);
        XPathNavigator navigator = doc.CreateNavigator();

        XPathExpression expr = navigator.Compile("//title");
        XPathNodeIterator iterator = expr.Select();

        Console.WriteLine("Book titles:");
        while (iterator.MoveNext())
        {
            Console.WriteLine(iterator.Current.Value);
        }
    }
}

輸出：

Book titles:
Book 1
Book 2
Book 3

2. 根據書籍ID選擇書籍：

using System;
using System.Xml;
using System.Xml.XPath;

class XPathExample
{
    static void Main()
    {
        string xml = @"
          <books>
            <book id='1'>
              <title>Book 1</title>
              <author>Author 1</author>
            </book>
            <book id='2'>
              <title>Book 2</title>
              <author>Author 2</author>
            </book>
            <book id='3'>
              <title>Book 3</title>
              <author>Author 3</author>
            </book>
          </books>";

        XPathDocument doc = new XPathDocument(xml);
        XPathNavigator navigator = doc.CreateNavigator();

        XPathExpression expr = navigator.Compile("//book[@id='2']");
        XPathNodeIterator iterator = expr.Select();

        Console.WriteLine("Book with ID 2:");
        while (iterator.MoveNext())
        {
            Console.WriteLine(iterator.Current.Value);
        }
    }
}

輸出：

Book with ID 2:
Book 2
Author 2

這些示例展示了如何使用C#和XPath表達式查詢XML文檔。您可以根據需要修改這些示例以滿足您的實際需求。