xml learning (6) in c#Xpath example-XML/RSS Tutorial-php.cn

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In c#, we often search and traverse nodes, this is what we can use XPath syntax, Example Xml:

<?xml version="1.0" encoding="utf-8" ?>  
<pets>  
<cat color="black" weight="10">  
<price>100</price>  
<desc>this is a black cat</desc>  
</cat>  
<cat color="white" weight="9">  
<price>80</price>  
<desc>this is a white cat</desc>  
</cat>  
<cat color="yellow" weight="15">  
<price>80</price>  
<desc>this is a yellow cat</desc>  
</cat>  
   
   
<dog color="black" weight="10">  
<price>100</price>  
<desc>this is a black dog</desc>  
</dog>  
<dog color="white" weight="9">  
<price>80</price>  
<desc>this is a white dog</desc>  
</dog>  
<dog color="yellow" weight="15">  
<price>80</price>  
<desc>this is a yellow dog</desc>  
</dog>  
</pets>

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XPath syntax:

1. Symbols in XPath

Symbol Description/Select the dog node under the pets node/pets/dog[0 ]/..# #@xx//dog/@color[…]//dog[@color='white']The number in the square brackets is the node index, similar to the array in languages such as C#, the array subscript starts from 1##| The single vertical bar indicates the combination of merged nodes//dog[@color='white'] | //cat[ @color='white']The asterisk represents a node or attribute with any name//dog/*

Example

Example description

means selecting from the root node

/pets

Select the root node pets

Represents the spacer between the node and the child node

/pets/dog

//xx

means searching from the entire xml document, regardless of the current node position

//price

Select all price nodes in the document

Single English A half-width period indicates selecting the current node

/pets/.

Selecting the pets node

##Double dot indicates selecting the parent node

represents the pets node, which is the parent node of the first dog node

represents the selection attribute

Indicates selecting the color attribute set of all dog nodes

中The brackets indicate the selection conditions, and the conditions inside the brackets are

All dogs with the color white Node

//dog[/price<100]

All dogs whose price byte point value is less than 100 Node

//dog[1]

The first dog node

//dog[ last()]

The last dog node, last() is the xPath built-in function

The dog node whose color attribute is white and the cat node whose color attribute is white

Represents all child nodes of the dog node

//dog/@*

Represents all attribute nodes of the dog node

2. XPath mathematical operators

+ The plus sign means adding

- means subtracting numbers

* means multiplying by

p means dividing by, where the mathematical division sign / has been used as a separator between nodes

mod means remainder

3. XPath logical operator

= equals, equivalent to ==

# in c

!= Not equal to

> Greater than

>= Greater than or equal to

< Less than

<= Less than or equal to

and and with the relationship

or or or the relationship

4. XPath Axes Literally translated this is XPath The meaning of axis, but according to my understanding, it is more appropriate to translate it into XPath node relationship operation keyword, which is a set of keywords plus :: double colon to indicate a node or a group of nodes that are related to the current node.

Use syntax: axisname::nodetest[predicate] That is, axis name::node name [get node condition]

The specific description is as follows:

##Quite For @weight, attribute:: and @ are equivalent childAll bytes of the current node Clickchild::*[name()!='price']Select the child node whose name is not pricedescendant Descendant Nodedescendant::*[@*] Descendant nodes with attributesdescendant-or-selfDescendant nodes and current nodesdescendant-or-self::* followingAll nodes after the current node in the Xml documentfollowing::*following-siblingThe same father and younger brother node of the current nodefollowing-sibling: :precedingAll nodes before the current node in the Xml documentpreceding::*##namespaceparentpreceding-siblingself

5. 常用的XPath函数介绍：

在XPath表达式中常用的函数有下面两个：

position() 表示节点的序号例如 //cat[position() = 2] 表示取序号为2的dog节点

last() 表示取最后一个节点 //cat[last()]

name() 表示当前节点名字 /pets/*[name() != 'pig'] 表示/pets下名字不是pig的子节点

XPath的函数还有很多，包括字符串函数，数字函数和时间函数等，具体可以参考w3的网站。

以上是XPath的语法，下面我们看下如何在.Net中使用XPath

在.Net中可以通过XPathDocument或者XmlDocument类使用XPath。XPathDocument是只读的方式定位Xml节点或者属性文本等，而XmlDocument则是可读写的。

如下代码示例展示了如何使用XPathDocument和XmlDocument。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.XPath;
using System.Xml;
 
namespace UseXPathDotNet
{
class Program
{
static void Main(string[] args)
{
UseXPathWithXPathDocument();
 
UseXPathWithXmlDocument();
 
Console.Read();
}
 
static void UseXPathWithXmlDocument()
{
XmlDocument doc = new XmlDocument();
doc.Load("http://www.cnblogs.com/yukaizhao/rss");
//使用xPath选择需要的节点
XmlNodeList nodes = doc.SelectNodes("/rss/channel/item[position()<=10]");
foreach (XmlNode item in nodes)
{
string title = item.SelectSingleNode("title").InnerText;
string url = item.SelectSingleNode("link").InnerText;
Console.WriteLine("{0} = {1}", title, url);
}
}
 
static void UseXPathWithXPathDocument()
{
XPathDocument doc = new XPathDocument("http://www.cnblogs.com/yukaizhao/rss");
XPathNavigator xPathNav = doc.CreateNavigator();
//使用xPath取rss中最新的10条随笔
XPathNodeIterator nodeIterator = xPathNav.Select("/rss/channel/item[position()<=10]");
while (nodeIterator.MoveNext())
{
XPathNavigator itemNav = nodeIterator.Current;
string title = itemNav.SelectSingleNode("title").Value;
string url = itemNav.SelectSingleNode("link").Value;
Console.WriteLine("{0} = {1}",title,url);
}
 
}
}
}

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XPath使用示例，请看下面的代码注释　

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Xml;
 
namespace UseXPath1
{
class Program
{
static void Main(string[] args)
{
string xml = @"<?xml version=""1.0"" encoding=""utf-8"" ?>
<pets>
<cat color=""black"" weight=""10"" count=""4"">
<price>100</price>
<desc>this is a black cat</desc>
</cat>
<cat color=""white"" weight=""9"" count=""5"">
<price>80</price>
<desc>this is a white cat</desc>
</cat>
<cat color=""yellow"" weight=""15"" count=""1"">
<price>110</price>
<desc>this is a yellow cat</desc>
</cat>
 
 
<dog color=""black"" weight=""10"" count=""7"">
<price>114</price>
<desc>this is a black dog</desc>
</dog>
<dog color=""white"" weight=""9"" count=""4"">
<price>80</price>
<desc>this is a white dog</desc>
</dog>
<dog color=""yellow"" weight=""15"" count=""15"">
<price>80</price>
<desc>this is a yellow dog</desc>
</dog>
 
<pig color=""white"" weight=""100"" count=""2"">
<price>8000</price>
<desc>this is a white pig</desc>
</pig>
</pets>";
 
using (StringReader rdr = new StringReader(xml))
{
XmlDocument doc = new XmlDocument();
doc.Load(rdr);
 
//取所有pets节点下的dog字节点
XmlNodeList nodeListAllDog = doc.SelectNodes("/pets/dog");
 
//所有的price节点
XmlNodeList allPriceNodes = doc.SelectNodes("//price");
 
//取最后一个price节点
XmlNode lastPriceNode = doc.SelectSingleNode("//price[last()]");
 
//用双点号取price节点的父节点
XmlNode lastPriceParentNode = lastPriceNode.SelectSingleNode("..");
 
//选择weight*count=40的所有动物，使用通配符*
XmlNodeList nodeList = doc.SelectNodes("/pets/*[@weight*@count=40]");
 
//选择除了pig之外的所有动物,使用name()函数返回节点名字
XmlNodeList animalsExceptPigNodes = doc.SelectNodes("/pets/*[name() != &#39;pig&#39;]");
 
 
//选择价格大于100而不是pig的动物
XmlNodeList priceGreaterThan100s = doc.SelectNodes("/pets/*[price p @weight >10 and name() != &#39;pig&#39;]");
foreach (XmlNode item in priceGreaterThan100s)
{
Console.WriteLine(item.OuterXml);
}
 
//选择第二个dog节点
XmlNode theSecondDogNode = doc.SelectSingleNode("//dog[position() = 2]");
 
//使用xpath ，axes 的 parent 取父节点
XmlNode parentNode = theSecondDogNode.SelectSingleNode("parent::*");
 
//使用xPath选择第二个dog节点前面的所有dog节点
XmlNodeList dogPresibling = theSecondDogNode.SelectNodes("preceding::dog");
 
//取文档的所有子孙节点price
XmlNodeList childrenNodes = doc.SelectNodes("descendant::price");
}
 
Console.Read();
}
}
}

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