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Get all possible XPath expressions with XPathNavigator class?

I already have an algorithm to retrieve the XPath expressions of an Xml:
Avoid recursion on this function XML related
However, it is imperfect, unsecure, and it needs a lot of additional decoration to format the obtained expressions.
( for an example of desired formatting I mean this: Get avaliable XPaths and its element names using HtmlAgilityPack )
Then, I recently discovered the XPathNavigator class, and to improve in any way the reliability of my current code, I would like to know if with the XPathNavigator class I could retrieve all the XPath exprressions of the Xml document, because that way my algorithm could be based in the efficiency of the .Net framework logic and their rules instead of the imperfect logic of a single programmer.
I search for a solution in C# or Vb.Net.
This is what I tried:
Dim xDoc As XDocument =
<?xml version="1.0"?>
<Document>
<Tests>
<Test>
<Name>A</Name>
<Value>0.01</Value>
<Result>Pass</Result>
</Test>
<Test>
<Name>A</Name>
<Value>0.02</Value>
<Result>Pass</Result>
</Test>
<Test>
<Name>B</Name>
<Value>1.01</Value>
<Result>Fail</Result>
</Test>
</Tests>
</Document>
Dim ms As New MemoryStream
xDoc.Save(ms, SaveOptions.None)
ms.Seek(0, SeekOrigin.Begin)
Dim xpathDoc As New XPathDocument(ms)
Dim xpathNavigator As XPathNavigator = xpathDoc.CreateNavigator
Dim nodes As XPathNodeIterator = xpathNavigator.Select(".//*")
For Each item As XPathNavigator In nodes
Console.WriteLine(item.Name)
Next
With that code I only managed to get this (undesired)kind of output:
Document
Tests
Test
Name
Value
Result
Test
Name
Value
Result
Test
Name
Value
Result
Test
Name
Value
Result
Is possibly to extract all the XPath expressions using the XPathNavigator class?.

No, that's not possible. There are many, many ways to select a particular node with XPath. You might settle on some notion of the "canonical" XPath for any given node, but even that sounds hard to specify, and XPathNavigator has no such notion built in to help you.

Get parent element based on value of child element

I have an XDocument object where I am trying to get the direct parent element based on a child element's value.
Getting the child element's value has been no issue, but I am struggling with finding the correct way to get only the parent element. Having not worked with XML much, I have a suspicion that the solution is simple and I am overthinking it.
Essentially, based on the below XML, if <Active>true</Active> then I want the direct parent element (i.e. <AlertNotification>) and no other elements.
Thank you in advance.
An example of the XML
<?xml version="1.0" encoding="utf-16"?>
<Policies xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLschema">
<PolicyID>1</PolicyID>
<EmailNotification>
<Active>false</Active>
</EmailNotification>
<AlertNotification>
<Active>true</Active>
</AlertNotification>
<AlarmEnabled>
<Active>false</Active>
</AlarmEnabled>
</Policies>

I thinks you should replace the utf-16 in the first line to utf-8. Then you may try this:
XDocument doc = XDocument.Load(your file);
var elements = doc.Descendants("Active")
.Where(i => i.Value == "true")
.Select(i => i.Parent);

Find Element when XPath is variable

I am trying to compose an algorithm that will take XML as input, and find a value associated with a particular element, but the position of the element within the XML body varies. I have seen many examples of using XDocument.Descendants() but most (if not all) the examples expect the structure to be consistent, and descendants well known. I presume I will need to recurse the XML to find this value, but before heading that way, ask the general population.
What is the best way to find an element in an XDocument when the path for the element may be different on each call? Just need the first occurrence found, not in any particular order. Can be first occurrence found by going wide, or by going deep.
For example, if I am trying to find the element "FirstName", and if the input XML for Call one looks like..
<?xml version="1.0"?>
<PERSON><Name><FirstName>BOB</FirstName></Name></PERSON>
and the next call looks like:
<?xml version="1.0"?>
<PERSONS><PERSON><Name><FirstName>BOB</FirstName></Name></PERSON></PERSONS>
What do you recommend? Is there a "Find" option in XDocument that I have not seen?
UPDATE:
Simple example above works with lazyberezovsky answer of XDocument.Descendents, but real XML does not. My problematic XML...
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<To s:mustUnderstand="1" xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">http://localhost:52087/Service1.svc</To>
<Action s:mustUnderstand="1" xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">http://tempuri.org/IService1/GetDataUsingDataContract</Action>
</s:Header>
<s:Body>
<GetDataUsingDataContract xmlns="http://tempuri.org/">
<composite xmlns:a="http://schemas.datacontract.org/2004/07/WcfService2" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<a:BoolValue>false</a:BoolValue>
<a:Name>
<a:FirstName>BOB</a:FirstName>
</a:Name>
<a:StringValue i:nil="true" />
</composite>
</GetDataUsingDataContract>
</s:Body>
</s:Envelope>
UPDATE:
lazyberezovsky helped immensely showing me how Descendents is supposed to work. Be careful of namespaces in XML. Lesson learned. Found another . article with similar issues..
Search XDocument using LINQ without knowing the namespace
Resolved using the following snippet...
var xdoc = XDocument.Parse(xml);
var name = (from p in xdoc.Descendants() where p.Name.LocalName == "FirstName" select p.Value).FirstOrDefault();

When you are using Descendants for finding first occurrence of element, you don't need to know
structure of file. Following code will work for both your cases:
var xdoc = XDocument.Load(path_to_xml);
var name = (string)xdoc.Descendants("FirstName").FirstOrDefault();
Same with XPath
var name = (string)xdoc.XPathSelectElement("//FirstName[1]");

Without knowing all the possible permutations of the XML document (which is very unusual by the way) I don't think anyone could hope to give you any worthwhile recommendations.

"Just need the first occurrence found, not in any particular order." I think Descendants do trick. Look at this:
string xml = #"<?xml version=""1.0""?>
<PERSONS>
<PERSON>
<Name>
<FirstName>BOB</FirstName>
</Name>
</PERSON>
</PERSONS>";
XDocument doc = XDocument.Parse(xml);
Console.WriteLine(string.Join(",", doc.Descendants("FirstName").Select(e =>(string)e)));
xml = #"<?xml version=""1.0""?>
<PERSON>
<Name>
<FirstName>BOB</FirstName>
</Name>
</PERSON>";
doc = XDocument.Parse(xml);
Console.WriteLine(string.Join(",", doc.Descendants("FirstName").Select(e =>(string)e)));

XmlElement.SelectNodes(..) - finds nothing.. Help?

Sorry to bother you with such a simple question, but I'm stuck here since an hour:
I have an xml file that looks something like this:
<?xml version="1.0" encoding="utf-8"?>
<aaa xmlns="http://blabla.com/xmlschema/v1">
<bbb>
<ccc>Foo</ccc>
</bbb>
<ddd x="y" />
<ddd x="xx" />
<ddd x="z" />
</aaa>
I'm trying to access the elements 'ddd' like this:
var doc = new XmlDocument();
doc.Load("example.xml");
foreach (XmlNode dddNode in doc.DocumentElement.SelectNodes("//ddd"))
{
// do something
Console.WriteLine(dddNode.Attributes["x"].Value);
}
At runtime the foreach loop is skipped because I don't get any nodes back from the .SelectNodes method. I breaked before the loop and had a look at the InnerXML, that looks fine, and I also tried all sorts of XPaths (like "//bbb" or "/aaa/ddd"), but only "/" seems to not return null.
This exact code worked for me before, now it does not. I suspect something with that namespace declaration in the aaa tag, but I couldn't figure out why this should cause problems. Or.. can you see anything I may be missing?

This is xml-namespaces. There is no ddd. There is, however, x:ddd where x is your alias to http://blabla.com/xmlschema/v1. You'll need to test with namespaces - for example:
var nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("x", "http://blabla.com/xmlschema/v1");
var nodes = doc.DocumentElement.SelectNodes("//x:ddd", nsmgr);
// nodes has 3 nodes
Note x in the above is arbitrary; there is no significance in x other than convenience.
This (rather inconveniently) means adding the namespace (or an alias, as above) into all of your xpath expressions.

How do I find a XML node by path in Linq-to-XML

If I get the path to a specific node as a string can I somehow easily find said node by using Linq/Method of the XElement ( or XDocument ).
There are so many different types of XML objects it would also be nice if as a added bonus you could point me to a guide on why/how to use different types.
EDIT: Ok after being pointed towards XPathSelectElement I'm trying it out so I can give him the right answer I can't quite get it to work though. This is the XML I'm trying out
<Product>
<Name>SomeName</Name>
<Type>SomeType</Type>
<Quantity>Alot</Quantity>
</Product>
and my code
string path = "Product/Name";
string name = xml.XPathSelectElement(path).Value;
note my string is coming from elsewhere so I guess it doesn't have to be literal ( at least in debug mode it looks like the one above). I've also tried adding / in front. It gives me a null ref.

Try using the XPathSelectElement extension method of XElement. You can pass the method an XPath expression to evaluate. For example:
XElement myElement = rootElement.XPathSelectElement("//Book[#ISBN='22542']");
Edit:
In reply to your edit, check your XPath expression. If your document only contains that small snippet then /Product/Name will work as the leading slash performs a search from the root of the document:
XElement element = document.XPathSelectElement("/Product/Name");
If there are other products and <Product> is not the root node you'll need to modify the XPath you're using.

You can also use XPathEvaluate
XDocument document = XDocument.Load("temp.xml");
var found = document.XPathEvaluate("/documents/items/item") as IEnumerable<object>;
foreach (var obj in found)
{
Console.Out.WriteLine(obj);
}
Given the following xml:
<?xml version="1.0" encoding="utf-8" ?>
<documents>
<items>
<item name="Jamie"></item>
<item name="John"></item>
</items>
</documents>
This should print the contents from the items node.