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Using XMLreader and xpath in large xml-file C#

So I have this rather large XML-file i need to parse and I don't want to load the whole file in memory. The XML looks something like this:
<root>
<node attrib ="true">
<child childattrib=1>
</child>
</node>
<node attrib ="false">
<child childattrib=1>
</child>
</node>
</root>
What I want to do is go through each node named node and see if the attribute matches my search-critera. And I want to do it using xpath.
I found Parse xml in c# : combine xmlreader and linq to xml which helps me isolate the node in question.
But I cant use xpath on the parent node. I guess I'll have to create an xmldocument and load the reader, but I cant get it to work the way I want to.

Attributes need double quotes around value(childattrib). Try following which is a combination of xml reader and xml linq. When reading large xml files always use xmlreader.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;
namespace ConsoleApplication74
{
class Program
{
const string FILENAME = #"c:\temp\test.xml";
static void Main(string[] args)
{
XmlReader reader = XmlReader.Create(FILENAME);
while (!reader.EOF)
{
if (reader.Name != "node")
{
reader.ReadToFollowing("node");
}
if (!reader.EOF)
{
XElement node = (XElement)XElement.ReadFrom(reader);
if ((Boolean)node.Attribute("attrib"))
{
}
}
}
}
}
}

C# Evaluate multiple Xpaths against an XmlDocument

I am writing a C# program where I would like to store a series of XPath statements as strings and evaluate them against an XMLDocument (or some other C# XML structure if there's a better one for this purpose) and store the resulting values in a dictionary / object.
My challenge is that my XPaths are not being able to be evaluated.
As a very simplified example, suppose this is my XML:
<root>
<a>
<child1 Id="Id1" Name="Name1" />
<child2 Id="Id2" Name="Name2" />
</a>
</root>
and, for example, one of my XPaths is:
//a/*[#Id='Id1']/name()
(Get the name of a's child element with the Id attribute = "Id1")
The simplified version of the code I'm trying to write to do this would be:
var xpath = #"//a/*[#Id='Id1']/name()";
var xml = #"<root><a><child1 Id='Id1' Name='Name1' /><child2 Id='Id2' Name='Name2' /></a></root>";
var doc = new XmlDocument();
doc.LoadXml(xml);
var navigator = doc.CreateNavigator();
string ChildName = (string)navigator.Evaluate(xpath);
but I am getting the error that my XPath has an invalid token - Which I'm assuming the be the name() portion.
Is there any way to accomplish this using direct XPath statements rather than traversing the tree?
Thanks!!

Pretty sure you just need to rearrange your XPath if I'm understanding you correctly. Try this:
name(//a/*[#Id='Id1'])

Using xml liinq :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;
namespace ConsoleApplication68
{
class Program
{
static void Main(string[] args)
{
string xml = "<root>" +
"<a>" +
"<child1 Id=\"Id1\" Name=\"Name1\" />" +
"<child2 Id=\"Id2\" Name=\"Name2\" />" +
"</a>" +
"</root>";
XDocument doc = XDocument.Parse(xml);
Dictionary<string, string> dict = doc.Descendants("a").FirstOrDefault().Elements().Where(x => x.Name.LocalName.StartsWith("child"))
.GroupBy(x => (string)x.Attribute("Id"), y => (string)y.Attribute("Name"))
.ToDictionary(x => x.Key, y => y.FirstOrDefault());
}
}
}

Iteration on XML tags in C#

How would code an iteration to loop through the parent tag on a xml file as below:
<collection>
<parent>
<child1>DHL</child1>
<child2>9000000131</child2>
<child3>ISS Gjøvik</child13>
<child4>ISS Gjøvik</child4>
<child5>ISS Gjøvik</child5>
<child6>9999000000136</child6>
</parent>
<parent>
<child1>DHL</child1>
<child2>9000000132</child2>
<child3>ISS Gjøvik</child13>
<child4>ISS Gjøvik</child4>
<child5>ISS Gjøvik</child5>
<child6>9999000000136</child6>
</parent>
<parent>
<child1>DHL</child1>
<child2>9000000134</child2>
<child3>ISS Gjøvik</child13>
<child4>ISS Gjøvik</child4>
<child5>ISS Gjøvik</child5>
<child6>9999000000136</child6>
</parent>
</collection>
I need to insert the value of child1 as the primary key into the DB.

Have you tried the XmlReader? What do you have so far? Please show us some code. Just a reminder, StackOverflow is a helpdesk, not a programming service.
I see DHL in one of the tags. If that refers to the postal delivery company, they have an API (SDK) that is easy to use from within .NET code..
If you want to use XML (de)serialization that I would suggest that you start reading the System.Xml.Serialization namespace documentation. Microsoft has provided more than enough documentation and examples.
Link to namespace docs: https://msdn.microsoft.com/en-us/library/system.xml.serialization(v=vs.110).aspx
Here are some examples that contains anything that you would need to deserialzation the xml document to a poco class:
https://msdn.microsoft.com/en-us/library/58a18dwa(v=vs.110).aspx

Assuming your xml is in the string variable xml:
var xdoc = XDocument.Parse(xml);
foreach (var parentEls in xdoc.Root.Elements("parent"))
{
string child1Value = parentEls.Element("child1").Value;
// your logic using child1 value
}
Note that your xml is malformed - <child3> is closed by </child13>.

Using xml linq to parse everything
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;
namespace ConsoleApplication1
{
class Program
{
const string FILENAME = #"c:\temp\test.xml";
static void Main(string[] args)
{
XDocument doc = XDocument.Load(FILENAME);
var results = doc.Descendants("parent").Select(x => new {
child1 = (string)x.Element("child1"),
child2 = (string)x.Element("child2"),
child3 = (string)x.Element("child3"),
child4 = (string)x.Element("child4"),
child5 = (string)x.Element("child5"),
child6 = (string)x.Element("child6")
}).ToList();
}
}
}

Retrieving specific data from XML file

Using LINQ to XML.
I have an XML file which looks like this:
<?xml version="1.0" encoding="utf-8"?>
<TileMap xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Title>title</Title>
<Abstract>Some clever text about this.</Abstract>
<SRS>OSGEO:41001</SRS>
<Profile>global-mercator or something</Profile>
</TileMap>
I can retrieve the <Title> from this with no problems by using this little piece of code:
string xmlString = AppDomain.CurrentDomain.BaseDirectory + #"Capabilities\" + name + ".xml";
string xmlText = File.ReadAllText(xmlString);
byte[] buffer = Encoding.UTF8.GetBytes(xmlText);
XElement element = XElement.Load(xmlString);
IEnumerable<XElement> title =
from el in element.Elements("Title")
select el;
foreach (XElement el in title)
{
var elementValue = el.Value;
}
However, this isn't very flexible because say I have an XML file that looks like this:
<?xml version="1.0" encoding="utf-8"?>
<RootObject xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Services>
<TileMapService>
<Title>title</Title>
<href>http://localhost/root</href>
</TileMapService>
</Services>
</RootObject>
It can't find <Title> but it finds <Services> (I presume) but since it's not called "Title" it just ignores it. I'm not very strong in working with XML. How would I go about making a method that looks through the XML and fetches me "Title" or however you'd implement this?

You're currently just looking at the child elements of the root element.
Instead, if you want to find all descendants, use Descendants.
Additionally, there's no point in using a query expression of from x in y select x (or rather, there's a very limited point in some cases, but not here). So just use:
var titles = element.Descendants("Title");
Personally I would actually use XDocument here rather than XElement - you have after all got a whole document, complete with XML declaration, not just an element.

Change your LINQ query to:
IEnumerable<XElement> title =
from el in element.Descendants("Title")
select el;
Elements returns only the immediate children, Descendants returns all descendant nodes instead.

Descendants will select all the "Title" elements irrespective of the level. Please use xpath to correctly locate the element
using System;
using System.Linq;
using System.Xml.Linq;
using System.Xml.XPath;
using System.IO;
public class Program
{
public static void Main()
{
string xmlFile = AppDomain.CurrentDomain.BaseDirectory + #"Capabilities\" + name + ".xml";
XElement xml=XElement.Load(xmlFile);
IEnumerable<XElement> titleElements = xml.XPathSelectElements("//Services/TileMapService/Title");
}
}

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
}