如何：查找相关元素 (XPath-LINQ to XML)

更新：November 2007

本主题演示如何在由其他元素的值所引用的属性上获取元素选择。

XPath 表达式为：

.//Customer[@CustomerID=/Root/Orders/Order[12]/CustomerID]

示例

本示例查找第 12 个 Order 元素，然后查找该订单的客户。

注意，在 .Net 中，对列表的索引是从零开始的。 在 XPath 谓词中，对节点集合的索引是从 1 开始的。 本示例反映了这种差别。

本示例使用下面的 XML 文档：示例 XML 文件：客户和订单 (LINQ to XML)。

XDocument co = XDocument.Load("CustomersOrders.xml");

// LINQ to XML query
XElement customer1 =
    (from el in co.Descendants("Customer")
     where (string)el.Attribute("CustomerID") ==
          (string)(co
              .Element("Root")
              .Element("Orders")
              .Elements("Order")
              .ToList()[11]
              .Element("CustomerID"))
    select el)
    .First();

// An alternate way to write the query that avoids creation
// of a System.Collections.Generic.List:
XElement customer2 =
    (from el in co.Descendants("Customer")
     where (string)el.Attribute("CustomerID") ==
          (string)(co
              .Element("Root")
              .Element("Orders")
              .Elements("Order")
              .Skip(11).First()
              .Element("CustomerID"))
    select el)
    .First();

// XPath expression
XElement customer3 = co.XPathSelectElement(
  ".//Customer[@CustomerID=/Root/Orders/Order[12]/CustomerID]");

if (customer1 == customer2 && customer1 == customer3)
    Console.WriteLine("Results are identical");
else
    Console.WriteLine("Results differ");
Console.WriteLine(customer1);

Dim co As XDocument = XDocument.Load("CustomersOrders.xml")

' LINQ to XML query
Dim customer1 As XElement = ( _
    From el In co...<Customer> _
    Where el.@CustomerID = co.<Root>.<Orders>.<Order>. _
        ToList()(11).<CustomerID>(0).Value _
    Select el).First()

' An alternate way to write the query that avoids creation
' of a System.Collections.Generic.List:
Dim customer2 As XElement = ( _
    From el In co...<Customer> _
    Where el.@CustomerID = co.<Root>.<Orders>.<Order>. _
        Skip(11).First().<CustomerID>(0).Value _
    Select el).First()

' XPath expression
Dim customer3 As XElement = co.XPathSelectElement _
    (".//Customer[@CustomerID=/Root/Orders/Order[12]/CustomerID]")

If customer1 Is customer2 And customer1 Is customer3 Then
    Console.WriteLine("Results are identical")
Else
    Console.WriteLine("Results differ")
End If
Console.WriteLine(customer1)

本示例生成以下输出：

Results are identical
<Customer CustomerID="HUNGC">
  <CompanyName>Hungry Coyote Import Store</CompanyName>
  <ContactName>Yoshi Latimer</ContactName>
  <ContactTitle>Sales Representative</ContactTitle>
  <Phone>(503) 555-6874</Phone>
  <Fax>(503) 555-2376</Fax>
  <FullAddress>
    <Address>City Center Plaza 516 Main St.</Address>
    <City>Elgin</City>
    <Region>OR</Region>
    <PostalCode>97827</PostalCode>
    <Country>USA</Country>
  </FullAddress>
</Customer>

请参见

概念

针对 XPath 用户的 LINQ to XML