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Retrieve processing instructions using XDocument

I have an XML document containing processing instructions. I know that, with the XmlDocument class, you can use
var node = xmlDoc.SelectSingleNode("processing-instruction('xml-stylesheet')") as XmlProcessingInstruction;
but I want to use XDocument. How can I do this?

This is how I access an XML file's nodes with the XDocument class.
However, you'll have to be more specific on what you want to do with it.
XDocument doc = XDocument.Load("filepath");
var node = doc.Nodes().OfType<XElement>().SingleOrDefault(n => n.Name == "node name");
var node_value = node.Value;
var node_descendants = node.Descendants();
UPDATE:
As you may have noticed there's no SelectSingleNode in XDocument, in fact, to retrieve the node you want you'll have to fetch it from the corresponding ienumerable collection, or alternatively from the predefined FirstNode, NextNode, PreviousNode, LastNode, but you cannot apply any filters to those. Therefore the only ways to retrieve ProcessingInstruction nodes are
var pI_nodes = doc.Nodes().OfType<XProcessingInstruction>();
And
var pI_nodes = (from node in doc.Nodes()
where node.NodeType == System.Xml.XmlNodeType.ProcessingInstruction
select node);
If you expect to retrieve several ProcessingInstructions and need to filter these as well, the equivalent to the node name would the Target property
var filtered_pIs = pI_nodes_1.Where(pI => pI.Target == "xml-stylesheet");
And as a final reminder the value of the processing instruction is stored in the Data property.
string pI_value = filtered_pIs.First().Data

Here is one way:
var node = xDoc.Root.Nodes().OfType<XProcessingInstruction>().First();

Ignoring case in SelectSingleNode Xpath not working.

I am trying the below sample to select a node by ignoring the case and the select single node retuns null.
XmlDocument doc = new XmlDocument();
doc.LoadXml("<root><CHILD1>c1</CHILD1><CHILD2>c2</CHILD2></root>");
var node = doc.SelectSingleNode("root");
string nodeXpath = string.Format("//*[translate(#key, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz') = '{0}']","child1");
node = node.SelectSingleNode(nodeXpath);
string innertext = node.InnerText;
Can someone help.

#key in XPath means a reference to an attribute named key. There is no such attribute in your XML. If you meant to match by element name then you're supposed to use name() or local-name() instead :
...
string xpath = "//*[translate(name(), 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz') = '{0}']";
string nodeXpath = string.Format(xpath,"child1");
...

You can use LINQ to Xml in little bid more readable way
XDocument doc = XDocument.Parse("<root><CHILD1>c1</CHILD1><CHILD2>c2</CHILD2></root>");
var singleNode =
doc.Root
.Elements()
.FirstOrDefault(element => element.Name.ToString().ToLower().Equals("child1"));
But notice that XML support different nodes where name can be case sensitive(for example "Node" and "node") and "searching" elements in "ignore case" way can lead to problems in the future.

I was working through this today and I used your solution. I just wrapped it in a function and call it whenever I need to match on an element name that's under the root. Works like a charm. Thanks!
private string GetNodeXpathCaseInsensitive(string value)
{
string xpath = String.Format("//*[translate(name(), 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz') = '{0}']", value.ToLower());
return xpath;
}

C# XML XPath Query Not working

I cant get my Xpath query to work though on paper it should be right. I even tried to get a single node without the attribute and could not even get this ...
What am I doing wrong ?
var trxXml = new XmlDocument();
trxXml.Load(InputTrxFile);
XmlElement root = trxXml.DocumentElement;
var unitTestResult = trxXml.GetElementsByTagName("UnitTestResult");
foreach (XmlElement runinfo in unitTestResult)
{
// Find failed tests, works fine then...
string TestName = runinfo.GetAttribute("testName"); // works fine
// Want to find equivalent TestDefinitions/UnitTest
/* Tried
TestRun/TestDefinitions/UnitTest[#name='thetest']
/TestRun/TestDefinitions/UnitTest[#name='thetest']
TestDefinitions/UnitTest[#name='thetest']
/TestDefinitions/UnitTest[#name='thetest']
UnitTest[#name='thetest']
variations with no attribute test JUST to get a node
Example http://www.csharp-examples.net/xml-nodes-by-attribute-value/
*/
var xpath = string.Format(#"/TestRun/TestDefinitions/UnitTest[#name='{0}']", TestName);
XmlNode node = trxXml.SelectSingleNode(xpath);
XmlNode node2 = root.SelectSingleNode(xpath);
// These all return null

Any query that begins with a / represents an absolute path, i.e. it is from the root of the document. It seems your UnitTestResult element (at the very least) encloses your TestRun elements.
To have your query take into account the current context, you need to reference the current context. This can be selected using ..
Secondly, your XML elements all have a namespace, and this needs to make up part of your query. A prefix needs to be added to a namespace manager, used in your query and the manager passed into the query method.
So, taking these together, define the prefix:
var manager = new XmlNamespaceManager(new NameTable());
manager.AddNamespace("t", "http://microsoft.com/schemas/VisualStudio/TeamTest/2010");
Change your query:
./t:TestRun/t:TestDefinitions/t:UnitTest[#name='{0}']
And pass the namespace manager to the method:
trxXml.SelectSingleNode(xpath, manager);
XPath and XmlDocument are pain, though. This would all be a lot more straightforward in LINQ to XML:
var doc = XDocument.Load(InputTrxFile);
var unitTestQuery =
from result in doc.Descendants(ns + "UnitTestResult")
let name = (string)result.Attribute("testName")
from unitTest in result.Descendants(ns + "UnitTest")
where (string)unitTest.Attribute("name") == name
select unitTest;
var unitTest = unitTestQuery.Single();

Selecting a node if the attribute is equal to a predefined string

I'm currently using a loop which gives me a variable, which then needs to be fed into an Xpath method to get me any nodes with an attribute equal to my variable. So far, I've learned that Xpath allows you to select a node from the XML document using
root.SelectNodes("Element[#Attribute='SpecificValue']")
However, I'd like to know if there's a way I can insert a predefined variable where the specific value, so I can grab a different set of nodes with each iteration of my loop.
For example something like this:
string attribValue= "test"
root.SelectNodes("Element[#Attribute = attribValue]")

Use string formatting:
string attribValue = "test";
string expression = String.Format("Element[#Attribute = '{0}']", attribValue);
root.SelectNodes(expression);

Using XML Linq
XDocument doc = new XDocument();
XElement root = (XElement)doc.FirstNode;
string attribValue= "test";
var results = root.Descendants("Element").Where(x => x.Attribute("Attribute").Value == attribValue).ToList();​

Best way test for XPath existence in an XML file?

Lately I've been using XPathDocument and XNavigator to parse an XML file for a given XPath and attribute. It's been working very well, when I know in advance what the XPath is.
Sometimes though, the XPath will be one of several possible XPath values, and I'd like to be able to test whether or not a given XPath exists.
In case I'm getting the nomenclature wrong, here's what I'm calling an XPath - given this XML blob:
<foo>
<bar baz="This is the value of the attribute named baz">
</foo>
I might be looking for what I'm calling an XPath of "//foo/bar" and then reading the attribute "baz" to get the value.
Example of the code that I use to do this:
XPathDocument document = new XPathDocument(filename);
XPathNavigator navigator = document.CreateNavigator();
XPathNavigator node = navigator.SelectSingleNode("//foo/bar");
if(node.HasAttributes)
{
Console.WriteLine(node.GetAttribute("baz", string.Empty));
}
Now, if the call to navigator.SelectSingleNode fails, it will return a NullReferenceException or an XPathException. I can catch both of those and refactor the above into a test to see whether or not a given XPath returns an exception, but I was wondering whether there was a better way?
I didn't see anything obvious in the Intellisense. XPathNavigator has .HasAttributes and .HasChildren but short of iterating through the path one node at a time, I don't see anything nicer to use.

If you've given valid XPath but it doesn't match anything, SelectSingleNode won't throw a NullReferenceException - it will just return null.
If you pass SelectSingleNode some syntactically invalid XPath, that's when it will throw an XPathException.
So normally, you'd just need to test whether the returned value was null or not.

var baz = navigator.SelectSingleNode("//foo/bar/#baz");
if (baz != null) Console.WriteLine(baz);

I think it is not good to create an XMLNode object by executing navigator.SelectSingleNode(...).
You have to use navigator.Evaluate() instead:
if (Convert.ToBoolean(navigator.Evaluate(#"boolean(//foo/bar)"))) {...}

From memory, may contain errors.
XDocument doc = XDocument.Load("foo.xml");
var att = from a in doc.Descendants("bar")
select a.Attribute("baz")
foreach (var item in att) {
if (item != null) { ... }
}

If node == null then node.HasAttributes will throw a NullReferenceException. This situation will occur when //foo/bar does not match any elements in the XML document.

var node = XDocument.Load(filename)
.Descendants("bar")
.SingleOrDefault(e=>e.Attribute("baz") != null);
if (node != null) Console.WriteLine(node.Attribute("baz").Value);

I would probably be more specific in my xpath.
var doc = XDocument.Load(fileName);
var results = from r in doc.XPathSelectElements("/foo/bar[count(#baz) > 0]")
select r.Attribute("baz");
foreach (String s in results)
Console.WriteLine(s);