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System.InvalidOperationException when modifiying value in an xml file in C#

So i... Have this snippet of code what writes to an existing xml file... the code to me is VERY simple...
XElement element;
XDocument xdoc = XDocument.Load(FileLoc);
element = xdoc.Elements(XName.Get("gold", "http://schemas.datacontract.org/2004/07/DumaLegend")).Single();
element.Value = Gold.Text;
Good Right? good! but why does it give out that error which means that it can't find the thing? it's a very valid thing....
here is the xml file:
<?xml version="1.0" encoding="utf-8"?>
<Save xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/DumaLegend">
<saveInfo>
<energyPieces>0</energyPieces>
<fullEnergyCells>4</fullEnergyCells>
<fullHearts>4</fullHearts>
<globalSwitches xmlns:d3p1="a">
<d3p1:switchList xmlns:d4p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays" />
</globalSwitches>
<gold>0</gold>
<hasBigFireball>false</hasBigFireball>
<hasCombo>false</hasCombo>
<hasCrossbow>false</hasCrossbow>
<hasDash>false</hasDash>
<hasDashUpgrade>false</hasDashUpgrade>
<hasDoubleJump>false</hasDoubleJump>
<hasFireball>false</hasFireball>
<hasHookshot>false</hasHookshot>
<hasInvisPot>false</hasInvisPot>
<hasSecondCombo>false</hasSecondCombo>
<hasShieldUpgrade>false</hasShieldUpgrade>
<hasSmallFireball>false</hasSmallFireball>
<heartPieces>0</heartPieces>
<heroPosOnMap>0</heroPosOnMap>
<heroTokens>0</heroTokens>
<itemSlot1 xmlns:d3p1="http://schemas.datacontract.org/2004/07/DumaLegend.Objects.Consumables" i:nil="true" />
<itemSlot2 xmlns:d3p1="http://schemas.datacontract.org/2004/07/DumaLegend.Objects.Consumables" i:nil="true" />
<lives>3</lives>
<worldsUnlocked>0</worldsUnlocked>
<worldsUnlockedOnMap>0</worldsUnlockedOnMap>
</saveInfo>
<saveSlot>0</saveSlot>
</Save>

Use xdoc.Descendants(XName.Get("gold", "http://schemas.datacontract.org/2004/07/DumaLegend")).

From the docs for Elements
Returns a filtered collection of the child elements of this element or document, in document order. Only elements that have a matching XName are included in the collection.
There is only one child elements of your document, and that is the Save element.
What you are looking for is at the path Save/saveInfo/gold. So you can either use Elements like this:
XNamespace ns = "http://schemas.datacontract.org/2004/07/DumaLegend";
var gold = doc.Elements(ns + "Save")
.Elements(ns + "saveInfo")
.Elements(ns + "gold")
.Single();
Or you can use Descendants, which will search all child elements recursively.
XNamespace ns = "http://schemas.datacontract.org/2004/07/DumaLegend";
var gold = doc.Descendants(ns + "gold").Single();

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");

Getting an XElement with a namespace via XPathSelectElements

I have an XML e.g.
<?xml version="1.0" encoding="utf-8"?>
<A1>
<B2>
<C3 id="1">
<D7>
<E5 id="abc" />
</D7>
<D4 id="1">
<E5 id="abc" />
</D4>
<D4 id="2">
<E5 id="abc" />
</D4>
</C3>
</B2>
</A1>
This is may sample code:
var xDoc = XDocument.Load("Test.xml");
string xPath = "//B2/C3/D4";
//or string xPath = "//B2/C3/D4[#id='1']";
var eleList = xDoc.XPathSelectElements(xPath).ToList();
foreach (var xElement in eleList)
{
Console.WriteLine(xElement);
}
It works perfectly, but if I add a namespace to the root node A1, this code doesn't work.
Upon searching for solutions, I found this one, but it uses the Descendants() method to query the XML. From my understanding, this solution would fail if I was searching for <E5> because the same tag exists for <D7>, <D4 id="1"> and <D4 id="2">
My requirement is to search if a node exists at a particular XPath. If there is a way of doing this using Descendants, I'd be delighted to use it. If not, please guide me on how to search using the name space.
My apologies in case this is a duplicate.

To keep using XPath, you can use something link this:
var xDoc = XDocument.Parse(#"<?xml version='1.0' encoding='utf-8'?>
<A1 xmlns='urn:sample'>
<B2>
<C3 id='1'>
<D7><E5 id='abc' /></D7>
<D4 id='1'><E5 id='abc' /></D4>
<D4 id='2'><E5 id='abc' /></D4>
</C3>
</B2>
</A1>");
// Notice this
XmlNamespaceManager nsmgr = new XmlNamespaceManager(new NameTable());
nsmgr.AddNamespace("sample", "urn:sample");
string xPath = "//sample:B2/sample:C3/sample:D4";
var eleList = xDoc.XPathSelectElements(xPath, nsmgr).ToList();
foreach (var xElement in eleList)
{
Console.WriteLine(xElement);
}

but it uses the Descendants() method to query the XML. From my understanding, this solution would fail if I was searching for because the same tag exists for , and
I'm pretty sure you're not quite understanding how that works. From the MSDN documentation:
Returns a filtered collection of the descendant elements for this document or element, in document order. Only elements that have a matching XName are included in the collection.
So in your case, just do this:
xDoc.RootNode
.Descendants("E5")
.Where(n => n.Parent.Name.LocalName == "B4");

Try this
var xDoc = XDocument.Parse("<A1><B2><C3 id=\"1\"><D7><E5 id=\"abc\" /></D7><D4 id=\"1\"><E5 id=\"abc\" /></D4><D4 id=\"2\"><E5 id=\"abc\" /></D4></C3></B2></A1>");
foreach (XElement item in xDoc.Element("A1").Elements("B2").Elements("C3").Elements("D4"))
{
Console.WriteLine(item.Element("E5").Value);//to get the value of E5
Console.WriteLine(item.Element("E5").Attribute("id").Value);//to get the value of attribute
}

Can't select nodes in XML document

My XML document is the Corpus.xml file in the TEI XML Brown Corpus
I want to select every s node in the document, so I can iterate over them and extract data.
The problem is that no matter which method I try, I cannot select the s nodes! I've tried:
xml.Root.Descendants("s")
xml.Root.XPathSelectElements("s")
XPathDocument
But nothing works. I get no errors. The result returned is an empty set!

Your XML documents has namespace declared:
<?xml version="1.0"?>
<teiCorpus xmlns="http://www.tei-c.org/ns/1.0"
xmlns:xi="http://www.w3.org/2001/XInclude">
<!-- content -->
</teiCorpus>
So you should use XNamespace to select elements:
var xdoc = XDocument.Load(path_to_xml);
XNamespace ns = "http://www.tei-c.org/ns/1.0";
var elements = xdoc.Descendants(ns + "s");
Take a look on Working with XML Namespaces for further information.

XML string to XML document

I have a whole XML document in a String which i need to convert to a XML document and parse tags in the document

This code sample is taken from csharp-examples.net, written by Jan Slama:
To find nodes in an XML file you can use XPath expressions. Method XmlNode.Selec­tNodes returns a list of nodes selected by the XPath string. Method XmlNode.Selec­tSingleNode finds the first node that matches the XPath string.
XML:
<Names>
<Name>
<FirstName>John</FirstName>
<LastName>Smith</LastName>
</Name>
<Name>
<FirstName>James</FirstName>
<LastName>White</LastName>
</Name>
</Names>
CODE:
XmlDocument xml = new XmlDocument();
xml.LoadXml(myXmlString); // suppose that myXmlString contains "<Names>...</Names>"
XmlNodeList xnList = xml.SelectNodes("/Names/Name");
foreach (XmlNode xn in xnList)
{
string firstName = xn["FirstName"].InnerText;
string lastName = xn["LastName"].InnerText;
Console.WriteLine("Name: {0} {1}", firstName, lastName);
}

Using Linq to xml
Add a reference to System.Xml.Linq
and use
XDocument.Parse(string xmlString)
Edit: Sample follows, xml data (TestConfig.xml)..
<?xml version="1.0"?>
<Tests>
<Test TestId="0001" TestType="CMD">
<Name>Convert number to string</Name>
<CommandLine>Examp1.EXE</CommandLine>
<Input>1</Input>
<Output>One</Output>
</Test>
<Test TestId="0002" TestType="CMD">
<Name>Find succeeding characters</Name>
<CommandLine>Examp2.EXE</CommandLine>
<Input>abc</Input>
<Output>def</Output>
</Test>
<Test TestId="0003" TestType="GUI">
<Name>Convert multiple numbers to strings</Name>
<CommandLine>Examp2.EXE /Verbose</CommandLine>
<Input>123</Input>
<Output>One Two Three</Output>
</Test>
<Test TestId="0004" TestType="GUI">
<Name>Find correlated key</Name>
<CommandLine>Examp3.EXE</CommandLine>
<Input>a1</Input>
<Output>b1</Output>
</Test>
<Test TestId="0005" TestType="GUI">
<Name>Count characters</Name>
<CommandLine>FinalExamp.EXE</CommandLine>
<Input>This is a test</Input>
<Output>14</Output>
</Test>
<Test TestId="0006" TestType="GUI">
<Name>Another Test</Name>
<CommandLine>Examp2.EXE</CommandLine>
<Input>Test Input</Input>
<Output>10</Output>
</Test>
</Tests>
C# usage...
XElement root = XElement.Load("TestConfig.xml");
IEnumerable<XElement> tests =
from el in root.Elements("Test")
where (string)el.Element("CommandLine") == "Examp2.EXE"
select el;
foreach (XElement el in tests)
Console.WriteLine((string)el.Attribute("TestId"));
This code produces the following output:
0002
0006

Depending on what document type you want you can use XmlDocument.LoadXml or XDocument.Load.

Try this code:
var myXmlDocument = new XmlDocument();
myXmlDocument.LoadXml(theString);