Develop Reference

Select a XML node based on a sibling node

I'm trying to select an XML node, where another child of the parent node contains a specific value.
The XML looks like this:
<?xml version="1.0" encoding="UTF-8" ?>
<AuthorIT>
<Objects>
<Media>don't care</Media>
<Style>don't care</Style>
<Book>don't care</Book>
<Topic>don't care</Topic>
<Topic>
<Object>
<Description>Performance Evidence</Description>
</Object>
<Text>This is what I want to select</Text>
</Topic>
</Objects>
</AuthorIT>
I'm using XPath in C#. My query at the moment looks like this: (but doesn't work, obviously)
docNav = new XPathDocument(localFile);
nav = docNav.CreateNavigator();
xPath = "//Topic[Object/Description = 'Performance Evidence']/Text";
string value = nav.SelectSingleNode(xPath).Value;
How do I get the contents of the Text node, from the Topic that has an Object/Description value of "Performance Evidence"?

You should first select the Description node containing your needle text, then move back to the common parent and select the Text nodes that are its children.
//Topic/Object/Description[text()='Performance Evidence']/../../Text/text()

As Kirill Polishchuk said in a comment, my XPath was correct.
What I left out of the example XML was the key to the solution... Namespace!
I found my answer on this other question: Using Xpath With Default Namespace in C#

SelectSingleNode in XML for select second element doesn't work in C#

I have problem to select second node from root element in following example xml code:
<?xml version="1.0"?>
<config>
<FirstNode>
<ShowBlahBlah>
</ShowBlahBlah>
</FirstNode>
<SecondNode>
<ShowBlahBlah>
</ShowBlahBlah>
</SecondNode>
</config>
and using this C# code to select SecondNode:
XmlDocument doc = new XmlDocument();
doc.LoadXml(sReadXML);
XmlNode sChangesLog = doc.SelectSingleNode("config").SelectSingleNode("//SecondNode").SelectSingleNode("//ShowBlahBlah")
XmlNodeList sChildNodes = sChangesLog.ChildNodes;
but it selecting first node and return its value!
how can I fix this problem?

You're using // at the start of each of your selections - which means "find descendant nodes starting at the root" (so the context is irrelevant). You could either do things in one step as per Jeffrey's answer, or use relative paths:
doc.SelectSingleNode("config")
.SelectSingleNode("SecondNode")
.SelectSingleNode("ShowBlahBlah")
Personally I'd use LINQ to XML instead, if at all possible:
var doc = XDocument.Parse(sReadXml);
var changes = doc.Root.Element("SecondNode").Element("ShowBlahBlah");
LINQ to XML is generally a much cleaner API than XmlDocument et al.

I want to read a single element of an XML file and output it to an aspx control

My XML looks like this:
<product id="1">
<name>A thing</name>
<description>This is what it's like</description>
</product>
I've been looking for an example that might look something like this:
string productID = 1;
XDocument productsXML = XDocument.Load("Products.xml");
for item in productsXML {
if (item.ID == productID) {
productNameLabel.Text = item.name;
}
}
The idea being, I'll be able to output a single xml child element to the .aspx page.
A thing
Does that make sense? I've been looking for examples for a few hours, and I'm starting to think that there is lot more scaffolding that I need, but since all the examples are so different, I'm not sure which one to follow!
So, how can I grab the contents of a single XML child element, and output it to the .aspx? Take this:
<product id="1">
<name>A thing</name>
<description>This is what it's like</description
</product>
and output this:
A thing

You can use XPath (in System.Xml.XPath)
string value = productsXML.XPathSelectElement("//product[#id='1']/name").Value;

In this case, I agree with L.B.'s answer since it is rather simple. If you like Linq (I noticed you were using XDocument, which is a Linq to XML object), here's an alternative solution:
XDocument productsXML = XDocument.Load("Products.xml");
string item = productsXML.Elements("product")
.Where(p => p.Attribute("id").Value == productID)
.First()
.Element("name").Value;
productsXML.Elements("product") This gets all product nodes in the document.
.Where(p => p.Attribute("id").Value == productID) This gets only the product node(s) with the id that matches your productiD.
.First() Since the .Where function returns a collection of nodes, this grabs the first one.
.Element("name").Value; This finds an element within the product node called name and returns its value.
For such a simple schema, the XPath seems much less verbose, but a bit harder to understand if you don't know XPath in the first place. Linq is a lot more lines of code (in this instance), but is a lot more readable if you don't know XPath.

CreatePathNavigator with XDocument in C# (xml with linq)

I have a question. Is it possible to navigate to an specific field in an xml and add elements to it?
I tried this:
XDocument doc = XDocument.Load("myxmldoc.xml");
doc.Element("Table").Element(Product).CreateNavigator().Add(new XElement("Lamp", "Lamp"));
Needless to say it didn't work. The thing is that I have an xml that looks like this:
<Table>
<Product>
<Chair/>
<Table/>
<ChessBoard/>
</Product>
<Product>
<Chair/>
<Table/>
<ChessBoard/>
</Product>
<Product>
<Chair/>
<Table/>
<ChessBoard/>
</Product>
</Table>
As you can see I have many Product tags and I want to be able to add the item Lamp to an specific node with linq. Every time I add elements with:
doc.Element("Table").Element("Porduct").Add(new XElement("Lamp", "Lamp"));
The Lamp element goes to the first Product node. I want it to go to an specific node, let's say the second one.
Someone advised me to use:
doc.Element("Table").Element("Porduct").ElementAt(2).Add(new XElement("Lamp", "Lamp"));
But it doesn't work. Visual Studio 2010 says it doesn't exist.

You need Elements("Porduct") (note the s) to use ElementAt() :
doc.Element("Table").Elements("Porduct").ElementAt(2).Add(...);
It would be a little easier if the Products had something to identify them, like an ID attribute.
(And probably you should fix the typo Porduct / Product)

XPath Query C# to pull out nodes dynamically

I am having to write a XPath Query to pull out the answer for a question based on the question id. The question id is passed dynamically to the query. I cannot use LINQ as the solution is in NET 2.0. Please find the XML file below
<?xml version="1.0" encoding="utf-8" ?>
<Questionaire>
<question id="1">
<answer>1</answer>
<correctAnswer>Text</correctAnswer>
</question>
<question id="2">
<answer>2</answer>
<correctAnswer>Text</correctAnswer>
</question>
</Questionaire>
I'am a novice to XPath and find it hard to get my head around it.
Many thanks in advance.

You could use the XmlDocument class and the SelectSingleNode method to perform XPath queries. You may checkout the following article for examples. In your case the XPath query will be something along the lines of Questionaire/question[id='1'] where the id could be variable of course in order to fetch the corresponding node. Once you find the <question> node corresponding to your search criteria you could navigate to its child nodes.

Your XPath expression can be dynamically generated like this:
myExpression = string.Format("/*/*[id='{0}']/answer", theId);
then, depending on the object representing the XML document you need to call one of the following methods: Select(), SelectNodes(), SelectSingleNode(), Evaluate().
Read the MSDN documentation about the appropriate methods of XmlDocument, XPathDocument, XPathNavigator and XPathExpression.