如何使用VB.NET从XML获取属性值？

Question

我有以下xml：

    <?xml version="1.0" encoding="UTF-8"?>
    <gesmes:Envelope xmlns:gesmes="http://www.gesmes.org/xml/2002-08-01" xmlns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref">
    <gesmes:subject>Reference rates</gesmes:subject>
    <gesmes:Sender>
    <gesmes:name>European Central Bank</gesmes:name>
    </gesmes:Sender>
    <Cube>
    <Cube time='2016-09-12'>
        <Cube currency='USD' rate='1.1226'/>
        <Cube currency='JPY' rate='114.38'/>
    </Cube>
    </Cube>
    </gesmes:Envelope>

我想获取每种货币的属性值。 现在，我正在使用它，但是它不起作用：

    Dim xmlTree1 As New XmlDocument()
    xmlTree1.Load("C:\\download\eurofxref-daily.xml")

    Dim currencyUSD As String = xmlTree1.SelectSingleNode("/gesmes:Envelope/Cube/Cube/Cube[@currency='USD']/@rate").Value
    Dim currencyJPY As String = xmlTree1.SelectSingleNode("/gesmes:Envelope/Cube/Cube/Cube[@currency='JPY']/@rate").Value

Answer 1

我正在使用以下代码从xml获取值，并且可以正常工作：

 Dim xmlTree1 As New XmlDocument()
 xmlTree1.Load("C:\\kursna_standalone\download\eurofxref-daily.xml")

 Dim xmlnsManager1 As New System.Xml.XmlNamespaceManager(xmlTree1.NameTable)
 xmlnsManager1.AddNamespace("gm1", "http://www.ecb.int/vocabulary/2002-08-01/eurofxref")

 Dim currencyUSD As String = xmlTree1.SelectSingleNode("//gm1:Cube/gm1:Cube/gm1:Cube[@currency='USD']/@rate", xmlnsManager1).Value
 Dim currencyJPY As String = xmlTree1.SelectSingleNode("//gm1:Cube/gm1:Cube/gm1:Cube[@currency='JPY']/@rate", xmlnsManager1).Value                                                                        

Answer 2

一种替代方法是使用VB.NET的很棒的Linq-to-XML功能（Framework 3.5和更高版本）：

Imports <xmsns:xref="http://www.ecb.int/vocabulary/2002-08-01/eurofxref">

...

Dim xdoc = XDocument.Load("C:\kursna_standalone\download\eurofxref-daily.xml")
Dim cubes = xdoc.Root.<xref:Cube>.<xref:Cube>
Dim currencyUSD = cubes.Where(Function(x) x.currency = "USD").@rate
Dim currencyJPY = cubes.Where(Function(x) x.currency = "JPY").@rate

_{（注意：我所有的VB代码示例均假定Option Strict和Option Infer处于活动状态）。}