Navigating by attribute in XPath

[jon cosby]

This code shows all values for the config node. I'm trying to retrieve
the value for systrayenabled for attrib "jon". Something wrong?


sXmlPath = Application.StartupPath.ToString() + "\\settings.xml";
XmlDocument xmlConfig = new XmlDocument();
xmlConfig.Load(sXmlPath);
XPathNavigator navigator = xmlConfig.CreateNavigator();

XPathNodeIterator iterator =
navigator.Select("timer/config[@user='jon']/systrayenabled");
MessageBox.Show(iterator.Current.Value);

 

[Michael Nemtsev]

Hello Jon,

What's the problem?
I'd be better if you showed you xml

JC> This code shows all values for the config node. I'm trying to
JC> retrieve the value for systrayenabled for attrib "jon". Something
JC> wrong?
JC>
JC> sXmlPath = Application.StartupPath.ToString() + "\\settings.xml";
JC> XmlDocument xmlConfig = new XmlDocument();
JC> xmlConfig.Load(sXmlPath);
JC> XPathNavigator navigator = xmlConfig.CreateNavigator();
JC> XPathNodeIterator iterator =
JC> navigator.Select("timer/config[@user='jon']/systrayenabled");
JC> MessageBox.Show(iterator.Current.Value);
JC>
---
WBR,
Michael Nemtsev :: blog: http://spaces.msn.com/laflour

"At times one remains faithful to a cause only because its opponents do not
cease to be insipid." (c) Friedrich Nietzsche

 

[Jon Cosby]

Hello Jon,

What's the problem?
I'd be better if you showed you xml

JC> This code shows all values for the config node. I'm trying to
JC> retrieve the value for systrayenabled for attrib "jon". Something
JC> wrong?
JC>
JC> sXmlPath = Application.StartupPath.ToString() + "\\settings.xml";
JC> XmlDocument xmlConfig = new XmlDocument();
JC> xmlConfig.Load(sXmlPath);
JC> XPathNavigator navigator = xmlConfig.CreateNavigator();
JC> XPathNodeIterator iterator =
JC> navigator.Select("timer/config[@user='jon']/systrayenabled");
JC> MessageBox.Show(iterator.Current.Value);
JC>
---
WBR,
Michael Nemtsev :: blog: http://spaces.msn.com/laflour

"At times one remains faithful to a cause only because its opponents do not
cease to be insipid." (c) Friedrich Nietzsche

The output is all of the values for the config nodes for all users. I
am trying to retrieve a single value for "jon".


<timer>
<config user="default">
<systrayenabled>false</systrayenabled>
<systraytimer>1</systraytimer>
<logevents>false</logevents>
</config>
<config user="jon">
<systrayenabled>true</systrayenabled>
<systraytimer>1</systraytimer>
<logevents>false</logevents>
</config>
</timer>

 

[Jon Skeet [C# MVP]]

The output is all of the values for the config nodes for all users. I
am trying to retrieve a single value for "jon".

I suspect you're making things harder than you need to by using
XPathNavigator. I find it's usually easier to use SelectSingleNode on
the document (or element). Here's a sample:

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

class Test
{
static void Main()
{
string xml = @"
<timer>
<config user='default'>
<systrayenabled>false</systrayenabled>
<systraytimer>1</systraytimer>
<logevents>false</logevents>
</config>
<config user='jon'>
<systrayenabled>true</systrayenabled>
<systraytimer>1</systraytimer>
<logevents>false</logevents>
</config>
</timer>
";
XmlDocument doc = new XmlDocument();
doc.LoadXml (xml);

XmlNode node = doc.SelectSingleNode("timer/config
[@user='jon']/systrayenabled");
Console.WriteLine (node.OuterXml);
}
}

Now, as to why it wasn't working for you: from the docs of
XPathNodeIterator:

<quote>
An XPathNodeIterator object returned by the XPathNavigator class is not
positioned on the first node in a selected set of nodes. A call to the
MoveNext method of the XPathNodeIterator class must be made to position
the XPathNodeIterator object on the first node in the selected set of
nodes.
</quote>

And indeed if you call MoveNext() before printing the result, you'll
find you get the results you expect. I'd still use SelectSingleNode
though

 

[Jon Cosby]

The output is all of the values for the config nodes for all users. I
am trying to retrieve a single value for "jon".

I suspect you're making things harder than you need to by using
XPathNavigator. I find it's usually easier to use SelectSingleNode on
the document (or element). Here's a sample:

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

class Test
{
static void Main()
{
string xml = @"
<timer>
<config user='default'>
<systrayenabled>false</systrayenabled>
<systraytimer>1</systraytimer>
<logevents>false</logevents>
</config>
<config user='jon'>
<systrayenabled>true</systrayenabled>
<systraytimer>1</systraytimer>
<logevents>false</logevents>
</config>
</timer>
";
XmlDocument doc = new XmlDocument();
doc.LoadXml (xml);

XmlNode node = doc.SelectSingleNode("timer/config
[@user='jon']/systrayenabled");
Console.WriteLine (node.OuterXml);
}
}

Now, as to why it wasn't working for you: from the docs of
XPathNodeIterator:

<quote>
An XPathNodeIterator object returned by the XPathNavigator class is not
positioned on the first node in a selected set of nodes. A call to the
MoveNext method of the XPathNodeIterator class must be made to position
the XPathNodeIterator object on the first node in the selected set of
nodes.
</quote>

And indeed if you call MoveNext() before printing the result, you'll
find you get the results you expect. I'd still use SelectSingleNode
though

That works, but I'm using the XPathNavigator to modify the xml
document. I can navigate with a "MoveToChild/MoveToNext" sequence, but
I can't query values.

 

[Jon Skeet [C# MVP]]

That works, but I'm using the XPathNavigator to modify the xml
document.

Any reason not to modify the DOM model directly?

I can navigate with a "MoveToChild/MoveToNext" sequence, but
I can't query values.

If you call MoveNext() with the code you provided, then
iterator.Current.Value prints out "true" which is what you want, isn't
it?

 

[Jon Cosby]

Any reason not to modify the DOM model directly?


If you call MoveNext() with the code you provided, then
iterator.Current.Value prints out "true" which is what you want, isn't
it?

Not exactly what I meant. I need something like

navigator.MoveToNode("timesynch/config[@user='jon']/systrayenabled");
navigator.SetValue("Okay");

 

[Jon Skeet [C# MVP]]

Not exactly what I meant. I need something like

navigator.MoveToNode("timesynch/config[@user='jon']/systrayenabled");
navigator.SetValue("Okay");

So why not use SelectSingleNode (specifying the text node) and then set
the value to "Okay"?

Anyway, if you *do* genuinely need to use XPathNavigator, calling
MoveNext() to start with after calling Select should set you straight.