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How to select child node with XPath

I'm trying to get values from a XML document using the iXF format, but I'm having some issues with the XPath syntax.
I have the following XML document
<SOAP_ENV:Envelope xmlns:NS2="http://www.ixfstd.org/std/ns/core/classBehaviors/links/1.0" xmlns:NS1="CATIA/V5/Electrical/1.0" xmlns:tns="IXF_Schema.xsd" xmlns:ixf="http://www.ixfstd.org/std/ns/core/1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:SOAP_ENV="http://schemas.xmlsoap.org/soap/envelope/" xsi:schemaLocation="IXF_Schema.xsd ElectricalSchema.xsd">
<SOAP_ENV:Body>
<ixf:object id="Electrical Physical System00000089.1" xsi:type="tns:Harness">
<tns:Name>Electrical Physical System00000089.1</tns:Name>
</ixf:object>
<ixf:object id="X10(1)//X11(1)" xsi:type="tns:Wire">
<tns:Name>X10(1)//X11(1)</tns:Name>
<NS1:Wire>
<NS1:Length>763,752mm</NS1:Length>
<NS1:Color>RD</NS1:Color>
<NS1:OuterDiameter>1,32mm</NS1:OuterDiameter>
</NS1:Wire>
</ixf:object>
</SOAP_ENV:Body>
</SOAP_ENV:Envelope>
And i'm trying to find all the Wire objects and get the Name and Length values with the following code.
XmlDocument xlDocument = new XmlDocument();
xlDocument.Load(importFile);
XmlNamespaceManager nsManager = new XmlNamespaceManager(xlDocument.NameTable);
nsManager.AddNamespace("tns", "IXF_Schema.xsd");
nsManager.AddNamespace("xsi", "http://www.w3.org/2001/XMLSchema-instance");
nsManager.AddNamespace("ixf", "http://www.ixfstd.org/std/ns/core/1.0");
nsManager.AddNamespace("NS1", "CATIA/V6/Electrical/1.0");
nsManager.AddNamespace("NS2", "http://www.ixfstd.org/std/ns/core/classBehaviors/links/1.0");
//Get all wire objects
XmlNodeList wires = xlDocument.SelectNodes("descendant::ixf:object[#xsi:type = \"tns:Wire\"]", nsManager);
foreach (XmlNode wire in wires)
{
string wireName;
string wireLength;
XmlNode node = wire.SelectSingleNode("./tns:Name", nsManager);
wireName = node.InnerText;
XmlNode node1 = wire.SelectSingleNode("./NS1:Wire/NS1:Length", nsManager);
wireLength = node1.InnerText;
}
I can get the wireName value without any problems but the Length element selection always returns 0 matches and I can not figure out why. I also tried to only select the Wire element using the same syntax as the Name element ./NS1:Wire but that also returns 0 matches.

Your XML declares
xmlns:NS1="CATIA/V5/Electrical/1.0"
^^
Your C# declares a different namespacem
nsManager.AddNamespace("NS1", "CATIA/V6/Electrical/1.0")
^^
Make sure both namespaces match exactly.
Regarding your comment asking about the use of version numbers in namespaces...
It is an unfortunately common but certainly not widely accepted practice to include a version number in an XML namespace. Realize that by doing so, you're effectively saying that every namespaced XML component (element or attribute) should now be considered to differ from its counterpart in the old namespace. This is rarely what you want.
See also
Should I use a Namespace of an XML file to identify its version
What are the best practices for versioning XML schemas?

Search XML element with xml namespace prefix C#

Update
I want to have an expression (XPath, or Regex Expression, similar) that can match an XML element with a particular namespace. For example, I want to locate the value of the link element (e.g. I need the http://url within <b:link>http://url</b:link>) shown below. However, the namespace prefix varies depending on different xml files as shown in cases 1-3.
Considering the allowed character for namespace prefix (e.g. is any character allowed/valid) , could anyone provide the solution (XPath, Regex Expression or similar?
Please note that because the xml file is unknown, thus, the namespace and prefix are unknown until runtime. Does it mean I cannot use this XDocument/XmlDocument, because it requires namespace to be known in the code.
Update
Case 1
<A xmlns:b="link">
<b:link>http://url
</b:link>
</A>
Case 2
<A xmlns="link">
<link>http://url
</link>
</A>
Case 3
<A xmlns:a123="link">
<a123:link>http://url
</a123:link>
</A>
Please note that the url within the link element could be any http url, and unknown until runtime.
Update
Please mark up my question.

You need to know the namespaces you will be dealing with and register them with an XmlNamespaceManager. Here is an example:
XmlDocument doc = new XmlDocument();
doc.LoadXml("<A xmlns:b='link'><b:Books /></A>");
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("b", "link");
XmlNodeList books = doc.SelectNodes("//b:Books", nsmgr);
And if you want to do this using XDocument, which I would recommend for its brevity, here is how:
XDocument xDoc = XDocument.Parse("<A xmlns:b='link'><b:Books /></A>");
XNamespace ns = "link";
var books = xDoc.Descendants(ns + "Books");
If you do not know the namespace(s) ahead of time, see this post which shows how to query across an XDocument using only the local name. Here's an example:
XDocument xDoc = XDocument.Parse("<A xmlns:b='link'><b:Books /></A>");
var books = xDoc.Descendants().Where(e => e.Name.LocalName.ToLower() == "books");

Use an XML parser, not a regex.
That being said, you could use:
<(?:(.+?):)?Books />
And the namespace would be in captured group 1.
In fact, I'd more strongly recommend you use
<(?:([^<>]+?):)?Books />
To prevent mistakes like matching over another set of XML tags (who would use <> in a namespace anyway?!)

How can I update XML element or attribute which its name contains ":" special character

I have XML file and the elements/attributes names have ":" character, how I can update its vales?
<?xml version="1.0" encoding="utf-8"?>
<?mso-application progid="InfoPath.Document" versionProgid="InfoPath.Document.3"?>
<my:ECR my:NoOfAutho="16" my:hideDetails="0" my:Type="ECR" my:NoOfVBUCMApprovales="10" >
<my:ECRNo>148</my:ECRNo>
<my:Stage>Approved</my:Stage>
<my:Details>
<my:ReasonForCR>Reason For CR</my:ReasonForCR>
<my:AreaAffected_Publications_VBUCM>false</my:AreaAffected_Publications_VBUCM>
<my:AreaAffected_Engineering>true</my:AreaAffected_Engineering>
<my:AreaAffected_Production>false</my:AreaAffected_Production>
<my:AreaAffected_CustomerSupport>true</my:AreaAffected_CustomerSupport>
<my:AreaAffected_VBUCMTest>false</my:AreaAffected_VBUCMTest>
</my:AreaAffectedVB_UCM>

Your XML sample is invalid as shown. The my prefix is not defined in the XML.
If your XML contained xmlns:my="schemas.microsoft.com/office/infopath/2003/myXSD/…" then the XML would at least have some hope of being valid.
For manipulating XML with namespaces in .NET code, consider using Linq XDocument instead of XmlDocument. I have found Linq's XNamespace and XName types to be much, much easier to use with the XDocument family of classes than the old style XmlDocument's rather clunky handling of namespaces.
Change your XML to add the xmlns:my attribute to the root element:
<my:MNO xmlns:my="schemas.microsoft.com/office/infopath/2003/myXSD/…" my:NoOfAutho="16" etc... >
In your C# code, add a reference to the Linq stuff to the top of your source file:
using System.Xml.Linq;
Then use code like this (not checked, may contain syntax typos) to load the xml and access the element:
XNamespace ns = "schemas.microsoft.com/office/infopath/2003/myXSD/…";
XName MNO_Name = ns + "MNO";
XDocument doc = XDocument.Load(path2);
XElement MNO_Element = doc.Root.Descendants(MNO_Name).Single();
You can then read or modify the properties, attributes, and children of the MNO element.
To read the value of <MNO>100</MNO>, use MNO_Element.Value.
To write a new value to the element, assign to the value property: MNO_Element.Value = "120";
.Single() asserts that there is exactly one node that matches the selection criteria, similar to the .SelectSingleNode() function of XmlDocument.
As you can see from this code, the name of the "my" namespace prefix in the XML document is immaterial to the code that processes the XML - it's the URI that the "my" prefix represents that is what is important. The prefix is just shorthand so the the XML writer doesn't have to write long and laborious URIs everywhere.
Writing your XML processing code to be agnostic of the XML namespace prefix is very important because the prefix name can (and will) vary from one XML doc to the next, but the namespace URI will be the same.

I don't understand what you mean by "how can I update its values", but it will likely help if you understand that these are XML namespaces.
I.E., my:ECRNo has a simple element name of ECRNo with a namespace prefix of my, which maps to a URN or a URL - which should be declared with a xmlns:my=... within the XML (either where it is declared, at a parent, or in the XML root element) - but isn't shown in the XML sample you provided here.
To update this using XmlNode, you need to use the overloaded SelectSingleNode method that accepts a XmlNamespaceManager as the 2nd argument. You then need to all the .AddNamespace method on the namespace manager to register the my prefix. This is detailed at http://msdn.microsoft.com/en-us/library/system.xml.xmlnode.selectsinglenode%28v=VS.90%29.aspx .

Colons are not valid characters in xml elements / attributes. They are namespaces.
Your line
<my:ECR my:NoOfAutho="16" my:hideDetails="0" my:Type="ECR" my:NoOfVBUCMApprovales="10" >
Properly references the my namespace already, so you should just be able to do this:
<my:ECR NoOfAutho="16" hideDetails="0" Type="ECR" NoOfVBUCMApprovales="10" >
And you should be fine?
You will also have to remove the my: from other places in the file particularly closing tags
</ReasonForCR>

Parsing XML document with XPath, C#

So I'm trying to parse the following XML document with C#, using System.XML:
<root xmlns:n="http://www.w3.org/TR/html4/">
<n:node>
<n:node>
data
</n:node>
</n:node>
<n:node>
<n:node>
data
</n:node>
</n:node>
</root>
Every treatise of XPath with namespaces tells me to do the following:
XmlNamespaceManager mgr = new XmlNamespaceManager(xmlDoc.NameTable);
mgr.AddNamespace("n", "http://www.w3.org/1999/XSL/Transform");
And after I add the code above, the query
xmlDoc.SelectNodes("/root/n:node", mgr);
Runs fine, but returns nothing. The following:
xmlDoc.SelectNodes("/root/node", mgr);
returns two nodes if I modify the XML file and remove the namespaces, so it seems everything else is set up correctly. Any idea why it work doesn't with namespaces?
Thanks alot!

As stated, it's the URI of the namespace that's important, not the prefix.
Given your xml you could use the following:
mgr.AddNamespace( "someOtherPrefix", "http://www.w3.org/TR/html4/" );
var nodes = xmlDoc.SelectNodes( "/root/someOtherPrefix:node", mgr );
This will give you the data you want. Once you grasp this concept it becomes easier, especially when you get to default namespaces (no prefix in source xml), since you instantly know you can assign a prefix to each URI and strongly reference any part of the document you like.

The URI you specified in your AddNamespace method doesn't match the one in the xmlns declaration.

If you declare prefix "n" to represent the namespace "http://www.w3.org/1999/XSL/Transform", then the nodes won't match when you do your query. This is because, in your document, the prefix "n" refers to the namespace "http://www.w3.org/TR/html4/".
Try doing mgr.AddNamespace("n", "http://www.w3.org/TR/html4/"); instead.

linq to xml - get rid of blank xmlns

I'm trying to get rid of empty namespace tags in my xml file. All of the solutions i've seen are based creating the xml from scratch. I have various xelements constructed from a previous xml. All I'm doing is
XElement InputNodes = XElement.Parse(InputXML);
m_Command = InputNodes.Element("Command");
and it adding the xmlns = "" everywhere. This is really infuriating. Thanks for any help.

There's a post on MSDN blogs that shows how to get around this (reasonably) easily. Before outputing the XML, you'll want to execute this code:
foreach (XElement e in root.DescendantsAndSelf())
{
if (e.Name.Namespace == string.Empty)
{
e.Name = ns + e.Name.LocalName;
}
}
The alternative, as the poster mentions, is prefixing every element name with the namespace as you add it, but this seems like a nicer solution in that it's more automated and saves a bit of typing.

Possibly it's this: Empty namespace using Linq Xml
This would indicate your document is in a different default namespace than the elements you add.

I think the second answer down on this post:
XElement Add function adds xmlns="" to the XElement
was very useful. Basically if you just do
XNamespace rootNamespace = doc.Root.Name.NamespaceName;
XElement referenceElement = new XElement(rootNamespace + "Reference");
That should solve it. So I guess you have to tell it not to worry about a special namespace when you are creating the element. Odd.