Simple xpath query with default namespace

J

Jason Mobarak

Hello --

I'm attempting to get a handle on how to do xpath queries with
System.Xml -- so far the biggest hurdle has been how to deal with a
default namespace. If I use the test xml:

<?xml version="1.0" encoding="utf-8" ?>
<thing xmlns="urn:thing-schema-v1">
<foo>foo thing</foo>
<bar>bar thing</bar>
<baz>baz thing</baz>
</thing>

....everything in the code below fails (as in no nodes are located, when
3 should be). However, if I remove the xmlns declaration from the
<thing> root element everything works fine. What am I doing wrong?

using System;
using System.Collections.Generic;
using System.Text;

namespace XmlTest
{
class Program
{
static void Main(string[] args)
{
const string szXPath = "/thing/*";
const string szFilename = @"..\..\test-wo-xmlns.xml";

System.Xml.XmlReader xmlReader = new
System.Xml.XmlTextReader(System.IO.File.OpenRead(szFilename));

System.Xml.XmlNamespaceManager xmlNsMgr = new
System.Xml.XmlNamespaceManager(xmlReader.NameTable);
xmlNsMgr.AddNamespace(String.Empty, "urn:thing-schema-v1");

System.Xml.XPath.XPathDocument xpath = new
System.Xml.XPath.XPathDocument(xmlReader);

System.Xml.XPath.XPathNavigator xpathNavi =
xpath.CreateNavigator();

System.Xml.XPath.XPathNodeIterator xpathQuery =
xpathNavi.Select(szXPath);
while(xpathQuery.MoveNext())
{
string szValue = xpathQuery.Current.Value;
System.Console.WriteLine(szValue);
}

System.Xml.XPath.XPathExpression xpathExpr =
xpathNavi.Compile(szXPath);
xpathExpr.SetContext(xmlNsMgr);

xpathQuery = xpathNavi.Select(xpathExpr);
while (xpathQuery.MoveNext())
{
string szValue = xpathQuery.Current.Value;
System.Console.WriteLine(szValue);
}

System.Xml.XmlDocument xmlDoc = new
System.Xml.XmlDocument();
xmlDoc.Load(szFilename);

System.Xml.XmlNamespaceManager xmlNsMgr2 = new
System.Xml.XmlNamespaceManager(xmlDoc.NameTable);
xmlNsMgr2.AddNamespace(String.Empty,
"urn:thing-schema-v1");

System.Xml.XmlNodeList xmlNodes =
xmlDoc.SelectNodes(szXPath);

foreach(System.Xml.XmlNode xmlNode in xmlNodes)
{
string szValue = xmlNode.Value;
System.Console.WriteLine(szValue);
}
}
}
}
 
J

Jason Mobarak

I don't quite understand why this works... but as many others have
probably discovered if you create a namespace manager like so:

System.Xml.XmlNamespaceManager xmlNsMgr = new
System.Xml.XmlNamespaceManager(xmlReader.NameTable);
xmlNsMgr.AddNamespace("q", "urn:thing-schema-v1");

....then use a query such as "/q:thing/*" then everything in the code
below works. Why is this?
 
G

Guest

Hi Jason,
namespaces in XML serve the same purpose as namespace in C#, they are a
way to distinguish between two items that happen to have the same identifier.
In XML you may have one XML definition that has a node called <Account
BankNo="123"> which refers to a Bank Account and another node that is also
called <Account Name="bob"> that refers to a Video Rental account, so if both
nodes are part of the same XML document how could you distinguish between
them? The namespace allows that since one could be part of the namespace
http://www.example.org/banking and another http://www.example.org/videorental
then when you have your XPath query you will need to tell XPath which Account
element you mean by prefixing the name with the namespace in your query.
Hence in your example because the node is part of a namespace you need to use
that information in your XPath query.


Mark.
--
http://www.markdawson.org


Jason Mobarak said:
I don't quite understand why this works... but as many others have
probably discovered if you create a namespace manager like so:

System.Xml.XmlNamespaceManager xmlNsMgr = new
System.Xml.XmlNamespaceManager(xmlReader.NameTable);
xmlNsMgr.AddNamespace("q", "urn:thing-schema-v1");

....then use a query such as "/q:thing/*" then everything in the code
below works. Why is this?

Hello --

I'm attempting to get a handle on how to do xpath queries with
System.Xml -- so far the biggest hurdle has been how to deal with a
default namespace. If I use the test xml:

<?xml version="1.0" encoding="utf-8" ?>
<thing xmlns="urn:thing-schema-v1">
<foo>foo thing</foo>
<bar>bar thing</bar>
<baz>baz thing</baz>
</thing>

...everything in the code below fails (as in no nodes are located, when
3 should be). However, if I remove the xmlns declaration from the
<thing> root element everything works fine. What am I doing wrong?

using System;
using System.Collections.Generic;
using System.Text;

namespace XmlTest
{
class Program
{
static void Main(string[] args)
{
const string szXPath = "/thing/*";
const string szFilename = @"..\..\test-wo-xmlns.xml";

System.Xml.XmlReader xmlReader = new
System.Xml.XmlTextReader(System.IO.File.OpenRead(szFilename));

System.Xml.XmlNamespaceManager xmlNsMgr = new
System.Xml.XmlNamespaceManager(xmlReader.NameTable);
xmlNsMgr.AddNamespace(String.Empty, "urn:thing-schema-v1");

System.Xml.XPath.XPathDocument xpath = new
System.Xml.XPath.XPathDocument(xmlReader);

System.Xml.XPath.XPathNavigator xpathNavi =
xpath.CreateNavigator();

System.Xml.XPath.XPathNodeIterator xpathQuery =
xpathNavi.Select(szXPath);
while(xpathQuery.MoveNext())
{
string szValue = xpathQuery.Current.Value;
System.Console.WriteLine(szValue);
}

System.Xml.XPath.XPathExpression xpathExpr =
xpathNavi.Compile(szXPath);
xpathExpr.SetContext(xmlNsMgr);

xpathQuery = xpathNavi.Select(xpathExpr);
while (xpathQuery.MoveNext())
{
string szValue = xpathQuery.Current.Value;
System.Console.WriteLine(szValue);
}

System.Xml.XmlDocument xmlDoc = new
System.Xml.XmlDocument();
xmlDoc.Load(szFilename);

System.Xml.XmlNamespaceManager xmlNsMgr2 = new
System.Xml.XmlNamespaceManager(xmlDoc.NameTable);
xmlNsMgr2.AddNamespace(String.Empty,
"urn:thing-schema-v1");

System.Xml.XmlNodeList xmlNodes =
xmlDoc.SelectNodes(szXPath);

foreach(System.Xml.XmlNode xmlNode in xmlNodes)
{
string szValue = xmlNode.Value;
System.Console.WriteLine(szValue);
}
}
}

}
Click to expand...
Click to expand...
 
J

Jason Mobarak

Thanks Mark. You're explanation makes sense. I guess I was confused
by the fact that I needed to introduce a prefix in to the my xpath
query (a prefix that's no actually in the xml) in order for the library
to allow me to query for something in a namespace, regardless of
whether it's the default namespace.

Hi Jason,
namespaces in XML serve the same purpose as namespace in C#, they are a
way to distinguish between two items that happen to have the same identifier.
In XML you may have one XML definition that has a node called <Account
BankNo="123"> which refers to a Bank Account and another node that is also
called <Account Name="bob"> that refers to a Video Rental account, so if both
nodes are part of the same XML document how could you distinguish between
them? The namespace allows that since one could be part of the namespacehttp://www.example.org/bankingand anotherhttp://www.example.org/videorental
then when you have your XPath query you will need to tell XPath which Account
element you mean by prefixing the name with the namespace in your query.
Hence in your example because the node is part of a namespace you need to use
that information in your XPath query.

Mark.
--http://www.markdawson.org

Jason Mobarak said:
I don't quite understand why this works... but as many others have
probably discovered if you create a namespace manager like so:
Click to expand...
System.Xml.XmlNamespaceManager xmlNsMgr = new
System.Xml.XmlNamespaceManager(xmlReader.NameTable);
xmlNsMgr.AddNamespace("q", "urn:thing-schema-v1");
Click to expand...
....then use a query such as "/q:thing/*" then everything in the code
below works. Why is this?
Click to expand...
Hello --
I'm attempting to get a handle on how to do xpath queries with
System.Xml -- so far the biggest hurdle has been how to deal with a
default namespace. If I use the test xml:
<?xml version="1.0" encoding="utf-8" ?>
<thing xmlns="urn:thing-schema-v1">
<foo>foo thing</foo>
<bar>bar thing</bar>
<baz>baz thing</baz>
</thing>
...everything in the code below fails (as in no nodes are located, when
3 should be). However, if I remove the xmlns declaration from the
<thing> root element everything works fine. What am I doing wrong?
using System;
using System.Collections.Generic;
using System.Text;
namespace XmlTest
{
class Program
{
static void Main(string[] args)
{
const string szXPath = "/thing/*";
const string szFilename = @"..\..\test-wo-xmlns.xml";
System.Xml.XmlReader xmlReader = new
System.Xml.XmlTextReader(System.IO.File.OpenRead(szFilename));
System.Xml.XmlNamespaceManager xmlNsMgr = new
System.Xml.XmlNamespaceManager(xmlReader.NameTable);
xmlNsMgr.AddNamespace(String.Empty, "urn:thing-schema-v1");
System.Xml.XPath.XPathDocument xpath = new
System.Xml.XPath.XPathDocument(xmlReader);
System.Xml.XPath.XPathNavigator xpathNavi =
xpath.CreateNavigator();
System.Xml.XPath.XPathNodeIterator xpathQuery =
xpathNavi.Select(szXPath);
while(xpathQuery.MoveNext())
{
string szValue = xpathQuery.Current.Value;
System.Console.WriteLine(szValue);
}
System.Xml.XPath.XPathExpression xpathExpr =
xpathNavi.Compile(szXPath);
xpathExpr.SetContext(xmlNsMgr);
xpathQuery = xpathNavi.Select(xpathExpr);
while (xpathQuery.MoveNext())
{
string szValue = xpathQuery.Current.Value;
System.Console.WriteLine(szValue);
}
System.Xml.XmlDocument xmlDoc = new
System.Xml.XmlDocument();
xmlDoc.Load(szFilename);
System.Xml.XmlNamespaceManager xmlNsMgr2 = new
System.Xml.XmlNamespaceManager(xmlDoc.NameTable);
xmlNsMgr2.AddNamespace(String.Empty,
"urn:thing-schema-v1");
System.Xml.XmlNodeList xmlNodes =
xmlDoc.SelectNodes(szXPath);
foreach(System.Xml.XmlNode xmlNode in xmlNodes)
{
string szValue = xmlNode.Value;
System.Console.WriteLine(szValue);
}
}
}
}
Click to expand...
Click to expand...
Click to expand...
 

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