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XDocument get element that defines it's own namespace

As question states. I have a xml document (below) and I need to get X_ScalarWebApi_DeviceInfo that defines namespace urn:schemas-sony-com:av. Unfortunately it results in an error: {System.NullReferenceException: Object reference not set to an instance of an object. I'm mainly interested in ServiceList element, but it doesn't work as well. Platform - Windows 10 mobile.
Any ideas?
<?xml version="1.0"?>
<root xmlns="urn:schemas-upnp-org:device-1-0">
<specVersion>
<major>1</major>
<minor>0</minor>
</specVersion>
<device>
<deviceType>urn:schemas-upnp-org:device:Basic:1</deviceType>
<friendlyName>ILCE-6000</friendlyName>
<manufacturer>Sony Corporation</manufacturer>
<manufacturerURL>http://www.sony.net/</manufacturerURL>
<modelDescription>SonyDigitalMediaServer</modelDescription>
<modelName>SonyImagingDevice</modelName>
<UDN>uuid:000000001000-1010-8000-62F1894EE7BE</UDN>
<serviceList>
<service>
<serviceType>urn:schemas-sony-com:service:ScalarWebAPI:1</serviceType>
<serviceId>urn:schemas-sony-com:serviceId:ScalarWebAPI</serviceId>
<SCPDURL/>
<controlURL/>
<eventSubURL/>
</service>
</serviceList>
<av:X_ScalarWebAPI_DeviceInfo xmlns:av="urn:schemas-sony-com:av">
<av:X_ScalarWebAPI_Version>1.0</av:X_ScalarWebAPI_Version>
<av:X_ScalarWebAPI_ServiceList>
<av:X_ScalarWebAPI_Service>
<av:X_ScalarWebAPI_ServiceType>guide</av:X_ScalarWebAPI_ServiceType>
<av:X_ScalarWebAPI_ActionList_URL>http://192.168.122.1:8080/sony</av:X_ScalarWebAPI_ActionList_URL>
<av:X_ScalarWebAPI_AccessType/>
</av:X_ScalarWebAPI_Service>
<av:X_ScalarWebAPI_Service>
<av:X_ScalarWebAPI_ServiceType>accessControl</av:X_ScalarWebAPI_ServiceType>
<av:X_ScalarWebAPI_ActionList_URL>http://192.168.122.1:8080/sony</av:X_ScalarWebAPI_ActionList_URL>
<av:X_ScalarWebAPI_AccessType/>
</av:X_ScalarWebAPI_Service>
<av:X_ScalarWebAPI_Service>
<av:X_ScalarWebAPI_ServiceType>camera</av:X_ScalarWebAPI_ServiceType>
<av:X_ScalarWebAPI_ActionList_URL>http://192.168.122.1:8080/sony</av:X_ScalarWebAPI_ActionList_URL>
<av:X_ScalarWebAPI_AccessType/>
</av:X_ScalarWebAPI_Service>
</av:X_ScalarWebAPI_ServiceList>
</av:X_ScalarWebAPI_DeviceInfo>
</device>
</root>
Ah, the code:
XDocument xDoc = XDocument.Parse(xml_text);
//var av = xDoc.Root.GetDefaultNamespace();//.Attribute("xmlns");//
XNamespace av = "urn:schemas-sony-com:av";
System.Diagnostics.Debug.WriteLine(av);
<XElement> api_list = (List<XElement>)xDoc.Element(av + "X_ScalarWebAPI_DeviceInfo").Elements();
==EDIT==
Well, both solutions were ok, so I'm upvoting one and marking as answer the other :P
It was mentioned that the solution using only a 'local name' might cause false positive search results, so to be safe I'm using the first one. Thanks for help!

Element only returns elements directly beneath the current node. You need the to specify the entire path (note there are multiple namespaces):
XNamespace ns = "urn:schemas-upnp-org:device-1-0";
XNamespace av = "urn:schemas-sony-com:av";
var api_list = xDoc.Root.Element(ns + "device")
.Element(av + "X_ScalarWebAPI_DeviceInfo").Elements();

Using Xml Linq
XDocument doc = XDocument.Load(FILENAME);
XElement x_ScalarWebAPI_DeviceInfo = doc.Descendants().Where(x => x.Name.LocalName == "X_ScalarWebAPI_DeviceInfo").FirstOrDefault();
XNamespace ns = x_ScalarWebAPI_DeviceInfo.Name.Namespace;

The issue is indeed the Element or Elements methods only search the direct child elements. You can use Decendants() but you will get a collection, so you have to do First(expression) to het a single one.
But in the expression you can use .Name.LocalName to skip the whole namespace thing and look for just the name of the element.
For this question:
XDocument xDoc = XDocument.Parse(xml_text);
XElement x_ScalarWebAPI_DeviceInfo = doc.Descendants().First(x.Name.LocalName == "X_ScalarWebAPI_DeviceInfo");

XML SelectNode() returns nothing. Why does namespace matter?

I have code to get the nodes of a root element:
xmlNodes = rootElement.SelectNodes("DefinitionName");
It's not returning nodes that exist. In the debugger, I can expand rootElement to find DefinitionName. Apparently the problem is the fact that the file has a namespace defined (see XML below). MSDN says that I have to do something like this to get nodes to return:
Note: This has nothing to do with my code. This is the example from MSDN:
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("ab", "http://www.lucernepublishing.com");
XmlNodeList nodelist = doc.SelectNodes("//ab:book", nsmgr);
I have two questions:
Why does the namespace matter? If I want a node, and it exists, just give it to me.
My app processes many XML files. How am I supposed to specify the namespace (nsmgr.AddNamespace())? Do I need to parse the file to get that first?
I can't help but feeling that I'm taking the long, angst-filled way of doing this.
This is the XML:
<?xml version="1.0" encoding="utf-8"?>
<SessionStateInfo xmlns:i="http://www.w3.org/2001/XMLSchema-instance"
z:Id="1" z:Type="Company.Apps.MoreHere.Session.SessionStateInfo"
z:Assembly="assembly info here"
xmlns:z="http://schemas.microsoft.com/2003/10/Serialization/"
xmlns="http://schemas.datacontract.org/2004/07/MoreHere.Session">
<CoaterNumber>25</CoaterNumber>
<DefinitionName z:Id="2">Two Line</DefinitionName>
<EnableManualMode>true</EnableManualMode>

<SessionStateInfo ....
xmlns="http://schemas.datacontract.org/2004/07/MoreHere.Session">
means that this element and all its descendants are in the http://schemas.datacontract.org/2004/07/MoreHere.Session namespace. Since unprefixed names in an XPath always refer to elements in no namespace, you will need to bind this URI to a prefix and use that prefix in your XPath, even though no prefix is in use in the document.
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("mhs", "http://schemas.datacontract.org/2004/07/MoreHere.Session");
xmlNodes = rootElement.SelectNodes("mhs:DefinitionName", nsmgr);
If you know that the element(s) you are looking for will always have the same local name but may or may not have a namespace (or may have different namespaces) then you can use XPath tricks like
rootElement.SelectNodes("*[local-name() = 'DefinitionName']");

It matters because if there's a namespace attached, then "DefinitionName" is not enough. Imagine you have been given a list of people, all with the first name John:
John Smith
John Jones
John Murphy
What you're doing is the equivalent of asking for "John," instead of "John Smith" for example.

This isn't exactly the answer to the question but it may be an alternative solution using XDocument
using System;
using System.Dynamic;
using System.Xml.Linq;
using Microsoft.CSharp.RuntimeBinder;
using System.Linq;
namespace ConsoleApplication8
{
class Program
{
static void Main(string[] args)
{
XDocument document = XDocument.Load("SessionStateInfo.xml");
XNamespace nameSpace = document.Root.GetDefaultNamespace();
XElement node = document.Descendants(nameSpace + "DefinitionName").FirstOrDefault();
if (node != null)
{
Console.WriteLine("Cool! XDocument rocks! value: {0}", node.Value);
}
else
{
Console.WriteLine("Spoot! Didn't find it!");
}
}
}
}
This seems to work if the default namespace is specified or not.

Get nodes from xml files

How to parse the xml file?
<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<sitemap>
<loc>link</loc>
<lastmod>2011-08-17T08:23:17+00:00</lastmod>
</sitemap>
<sitemap>
<loc>link</loc>
<lastmod>2011-08-18T08:23:17+00:00</lastmod>
</sitemap>
</sitemapindex>
I am new to XML, I tried this, but it seems to be not working :
XmlDocument xml = new XmlDocument(); //* create an xml document object.
xml.Load("sitemap.xml");
XmlNodeList xnList = xml.SelectNodes("/sitemapindex/sitemap");
foreach (XmlNode xn in xnList)
{
String loc= xn["loc"].InnerText;
String lastmod= xn["lastmod"].InnerText;
}

The problem is that the sitemapindex element defines a default namespace. You need to specify the namespace when you select the nodes, otherwise it will not find them. For instance:
XmlDocument xml = new XmlDocument();
xml.Load("sitemap.xml");
XmlNamespaceManager manager = new XmlNamespaceManager(xml.NameTable);
manager.AddNamespace("s", "http://www.sitemaps.org/schemas/sitemap/0.9");
XmlNodeList xnList = xml.SelectNodes("/s:sitemapindex/s:sitemap", manager);
Normally speaking, when using the XmlNameSpaceManager, you could leave the prefix as an empty string to specify that you want that namespace to be the default namespace. So you would think you'd be able to do something like this:
// WON'T WORK
XmlDocument xml = new XmlDocument();
xml.Load("sitemap.xml");
XmlNamespaceManager manager = new XmlNamespaceManager(xml.NameTable);
manager.AddNamespace("", "http://www.sitemaps.org/schemas/sitemap/0.9"); //Empty prefix
XmlNodeList xnList = xml.SelectNodes("/sitemapindex/sitemap", manager); //No prefixes in XPath
However, if you try that code, you'll find that it won't find any matching nodes. The reason for this is that in XPath 1.0 (which is what XmlDocument implements), when no namespace is provided, it always uses the null namespace, not the default namespace. So, it doesn't matter if you specify a default namespace in the XmlNamespaceManager, it's not going to be used by XPath, anyway. To quote the relevant paragraph from the Official XPath Specification:
A QName in the node test is expanded into an expanded-name using the
namespace declarations from the expression context. This is the same
way expansion is done for element type names in start and end-tags
except that the default namespace declared with xmlns is not used: if
the QName does not have a prefix, then the namespace URI is null (this
is the same way attribute names are expanded). It is an error if the
QName has a prefix for which there is no namespace declaration in the
expression context.
Therefore, when the elements you are reading belong to a namespace, you can't avoid putting the namespace prefix in your XPath statements. However, if you don't want to bother putting the namespace URI in your code, you can just use the XmlDocument object to return the URI of the root element, which in this case, is what you want. For instance:
XmlDocument xml = new XmlDocument();
xml.Load("sitemap.xml");
XmlNamespaceManager manager = new XmlNamespaceManager(xml.NameTable);
manager.AddNamespace("s", xml.DocumentElement.NamespaceURI); //Using xml's properties instead of hard-coded URI
XmlNodeList xnList = xml.SelectNodes("/s:sitemapindex/s:sitemap", manager);

Sitemap has 2 sub nodes "loc" and "lastmod". The nodes that you are accessing are "name" and "url". that is why you are not getting any result. Also in your XML file the last sitemap tag is not closed properly with a corresponding Kindly try xn["loc"].InnerText and see if you get the desired result.

I would definitely use LINQ to XML instead of the older XmlDocument based XML API. You can accomplish what you are looking to do using the following code. Notice, I changed the name of the element that I am trying to get the value of to 'loc' and 'lastmod', because this is what is in your sample XML ('name' and 'url' did not exist):
XElement element = XElement.Parse(XMLFILE);
IEnumerable<XElement> list = element.Elements("sitemap");
foreach (XElement e in list)
{
String LOC= e.Element("loc").Value;
String LASTMOD = e.Element("lastmod").Value;
}

Use XPath to find a node by name attribute value

I am trying to find a node by the name attribute value.
Here is a sample of the xml document:
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE kfx:XMLRELEASE SYSTEM "K000A004.dtd">
<kfx:XMLRELEASE xmlns:kfx="http://www.kofax.com/dtd/">
<kfx:KOFAXXML>
<kfx:BATCHCLASS ID="00000008" NAME="CertficateOfLiability">
<kfx:DOCUMENTS>
<kfx:DOCUMENT DOCID="00000006" DOCUMENTCLASSNAME="COL">
<kfx:DOCUMENTDATA>
<kfx:DOCUMENTFIELD NAME="Producer Name" VALUE="Howalt+McDowell Insurance" />
...
....
Here is my attempted expression:
var xml = XDocument.Load(new StreamReader("C:\\Users\\Matthew_cox\\Documents\\test.xml"));
XNamespace ns = "http://www.kofax.com/dtd/";
XmlNamespaceManager nsm = new XmlNamespaceManager(xml.CreateNavigator().NameTable);
nsm.AddNamespace("kfx", ns.NamespaceName);
var docs = xml.Descendants(ns + "DOCUMENT");
foreach(var doc in docs)
{
doc.XPathSelectElement("/DOCUMENTDATA/DOCUMENTFIELD/[#name='Producer Name']", nsm); //this line produces this exception: Expression must evaluate to a node-set.
}

XML is case-sensitive. In provided XML kfx:DOCUMENTFIELD has NAME attribute. Also your XPath doesn't have reference to namespace.
Try this XPath:
kfx:DOCUMENTDATA/kfx:DOCUMENTFIELD[#NAME = 'Producer Name']

I see two things wrong.
First of all you are selecting starting with "/", this selects from the document root, so strip the leading slash.
Secondly the expression is a bit wierd. I would include the condition directly on DOCUMENTFIELD. (I am unsure if no expression on the node axis actually means something. As in is .../[..] equivalent to .../node()[..] or perhaps even .../*[..]?)
As Kirill notes, you should also watch the casing and namespaces, but this should solve c# complaining about expressions not evaluating to node sets:
kfx:DOCUMENTDATA/kfx:DOCUMENTFIELD[#NAME = 'Producer Name']

Reading XML with XDocument problem

I have a following XML (part of a .rdl report):
<Report xmlns:rd="http://schemas.microsoft.com/SQLServer/reporting/reportdesigner" xmlns="http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition">
<DataSources>
<DataSource Name="TMSSharepointDataSource">
<DataSourceReference>TMSSharepointDataSource</DataSourceReference>
<rd:DataSourceID>f06ffa33-238f-4d83-adfe-1eaa8df96e90</rd:DataSourceID>
</DataSource>
</DataSources>
</Report>
I try to parse and read it using following code:
byte[] fileContent = File.ReadAllBytes(#"path");
UTF8Encoding unicode = new UTF8Encoding();
string stringContent = unicode.GetString(fileContent);
XDocument xml = XDocument.Parse(stringContent);
XElement dsNode = xml.Root.Element("DataSources");
I can't figure out why is dsNode always null?

It's a namespace issue... you need to specify the namespace for the DataSources element. Fortunately, LINQ to XML makes this really easy:
XNamespace ns = "http://schemas.microsoft.com/sqlserver/" +
"reporting/2008/01/reportdefinition";
XElement dsNode = xml.Root.Element(ns + "DataSources");
Note the xmlns="http://..." part of the root element, which means that element and all elements below it which don't have an explicit namespace inherit that namespace.

You are probably missing a namespace reference. Your DataSources will inherit the namespace of the Report node and you will need both the namespace and element local name to generate an XName.
Alternatively you can do the following and skip the namespace check:
XElement dsNode =
xml
.Root
.DescendantNodes()
.Where(e => e.Name.LocalName.Equals("DataSources"))
.First();
This will return the first node where the local name is DataSources. In your example this will be the DataSources element.
Also, your loading of the document is very clumsy. I would suggest the following instead:
XDocument xml = XDocument.Load(File.OpenRead(#"path"));