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Strip Out XML from string

I am calling some XMLRPC API and sometimes I am getting some "dirty" response. Dirty response means that along with XML is returned some other content/html like:
<div>Some Html maybe> Or some additional string is here
<?xml version="1.0" encoding="ISO-8859-1"?>
<methodResponse>
<params>
<param>
<value><int>30</int></value>
</param>
</params>
</methodResponse>
I need a way to throw out anything what is not XML and read only XML from string response so from response above I get only:
<?xml version="1.0" encoding="ISO-8859-1"?>
<methodResponse>
<params>
<param>
<value><int>30</int></value>
</param>
</params>
</methodResponse>
If nothing it would be helpful if someone provide code which cleans only HTML and left only XML at least. Prefer code in C#

Try using a variation of this or possibly use XSLT to filter the response you get back from the API with something similar to this. XSLT is actually pretty powerful stuff when filtering XML. I know Visual Studios didn't support XSLT V.2, but if you could use V.2 in another editor its quiet useful.

How to remove the header from an XML document

I'm not exactly sure if it's called a header, but anyway, I'm using a serializer in C# to generate XML tags,
How do I remove the xml link that's stuck on my first tag, for example currently I have:
<FirstTag xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> testData
</FirstTag>
And this is the desired output:
<FirstTag> testData </FirstTag>

Linq to XML - how do I get this element value

Fairly simple one, but my knowledge is limited in this area. I'm using the following c# code to access the value of elements within my SGML and XML documents.
It's working fine when there is only one element with the given name in the document, but as soon as there are more than one element with the same name it throws an exception, obviously!
I need to use xpath or some other way of specifying the location of the element i'm trying to get the value of.
XDocument doc = XDocument.Load(sgmlReader);
string system = doc.Descendants("chapnum").Single().Value;
return system;
This works fine, if there is only one "chapnum" in the doc, but i need to specifically get the value of "chapnum" at the following nested location "dmaddres/chapnum".
How please?
Here is a sample of the xml doc. I'm trying to get the value of the "chapnum" element nested in the "dmaddress" element.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE dmodule []>
<dmodule xmlns:dc="http://www.purl.org/dc/elements/1.1/"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://www.s1000d.org/S1000D_2-3-1/xml_schema_flat/descript.xsd">
<idstatus>
<dmaddres>
<dmc><avee><modelic>xx</modelic><sdc>A</sdc><chapnum>29</chapnum>
<section>1</section><subsect>3</subsect><subject>54</subject><discode
>00</discode><discodev>AAA</discodev><incode>042</incode><incodev
>A</incodev><itemloc>D</itemloc></avee></dmc>
<dmtitle><techname>Switch</techname><infoname>Description of function</infoname>
</dmtitle>
<issno inwork="00" issno="001" type="new"/>
<issdate day="20" month="07" year="2012"/>
<language language="sx"/></dmaddres>
<status>
<security class="01"/><datarest><instruct><distrib>-</distrib><expcont
>Obey the national regulations for export control.</expcont></instruct>
<inform><copyright><para><refdm><avee><modelic>xx</modelic><sdc>A</sdc>
<chapnum>29</chapnum><section>1</section><subsect>3</subsect><subject
>54</subject><discode>00</discode><discodev>ZZZ</discodev><incode
>021</incode><incodev>Z</incodev><itemloc>D</itemloc></avee></refdm
></para></copyright><datacond>BREXREF=AJ-A-00-00-00-05ZZZ-022Z-D VERSUB=CDIM-V6</datacond>
</inform></datarest>
<rpc>xxxxx</rpc>
<orig>xxxxx</orig>
<applic>
<type>-</type>
<model model="xxxxx"><mfc>xxxxx</mfc><pnr>xxxxxxx</pnr></model>
</applic>
<brexref><refdm><avee><modelic>xx</modelic><sdc>A</sdc><chapnum>00</chapnum>
<section>0</section><subsect>0</subsect><subject>00</subject><discode
>05</discode><discodev>ZZZ</discodev><incode>022</incode><incodev
>Z</incodev><itemloc>D</itemloc></avee></refdm></brexref>

like this?
string system = doc.Descendants("dmaddres")
.Single(e => e.Element("chapnum") != null)
.Element("chapnum").Value;
string system = doc.Root.Element("dmaddres").Element("chapnum").Value;
would probably do just as well.

Change an XML node value

I have an xml document that looks like this
<?xml version="1.0"?>
<XML>
<VIDEO>
<WIDTH>800</WIDTH>
<HEIGHT>600</HEIGHT>
<COLORBITS>32</COLORBITS>
<GAMMA>255</GAMMA>
<FULLSCREEN>TRUE</FULLSCREEN>
<REFLECTION>true</REFLECTION>
<LIGHTMAP>true</LIGHTMAP>
<DYNAMICLIGHT>true</DYNAMICLIGHT>
<SHADER>true</SHADER>
<CHARACTORTEXTURELEVEL>0</CHARACTORTEXTURELEVEL>
<MAPTEXTURELEVEL>0</MAPTEXTURELEVEL>
<EFFECTLEVEL>0</EFFECTLEVEL>
<TEXTUREFORMAT>1</TEXTUREFORMAT>
<NHARDWARETNL>false</NHARDWARETNL>
</VIDEO>
</XML>
I want to change the value of the "MAPTEXTURELEVEL" node from 0 to 6 using the checked statement of a checkbox in a C# application, but I really have no idea of how I can do it.

I don't have VS to test it, but it should be something like this using LINQ to XML:
var doc = XDocument.Load("video.xml");
doc
.Element("XML")
.Element("VIDEO")
.SetElementValue("MAPTEXTURELEVEL", 6);
doc.Save("video_modified.xml");
Hope it helps!

In XML, what are the nodes with question marks called, and how do I add them in C#?

Here's an example of an XML file created in InfoPath:
<?xml version="1.0" encoding="UTF-8"?>
<?mso-infoPathSolution solutionVersion="1.0.0.1" productVersion="12.0.0" PIVersion="1.0.0.0" href="file:///C:\Metastorm\Sample%20Procedures\InfoPath%20samples\Template1.xsn" name="urn:schemas-microsoft-com:office:infopath:Template1:-myXSD-2010-07-21T14-21-13" ?>
<?mso-application progid="InfoPath.Document" versionProgid="InfoPath.Document.2"?>
<my:myFields xmlns:my="http://schemas.microsoft.com/office/infopath/2003/myXSD/2010-07-21T14:21:13" xml:lang="en-us">
<my:field1>hello</my:field1>
<my:field2>world</my:field2>
</my:myFields>
What are those top 3 nodes with the question mark called... and how do I create them in C#?
So far I have this:
XmlDocument xmldoc;
XmlDeclaration xmlDeclaration;
xmldoc=new XmlDocument();
xmlDeclaration = xmldoc.CreateNode(XmlNodeType.XmlDeclaration, "", "") as XmlDeclaration;
xmlDeclaration.Encoding = "UTF-8";
xmldoc.AppendChild(xmlDeclaration);
This works fine for the top XML declaration node , but how do I create the next two?
Thanks in advance :)

These are called processing instructions. Add 'em using XmlDocument.CreateProcessingInstruction.

Those are called processing instructions. You can use the XmlProcessingInstruction class to interact with them in an XmlDocument.
As with most elements defined within an XmlDocument, you cannot instantiate it directly; you must use the appropriate factory method on XmlDocument (CreateProcessingInstruction in that particular case.)

Thanks for explaining that these are processing instructions. Using CreateProcessingInstruction as suggested, here is the solution:
xmlPi = xmldoc.CreateProcessingInstruction("mso-infoPathSolution", "solutionVersion=\"1.0.0.1\" productVersion=\"12.0.0\" PIVersion=\"1.0.0.0\" href=\"file:///C:\\Metastorm\\Sample%20Procedures\\InfoPath%20samples\\Template1.xsn\" name=\"urn:schemas-microsoft-com:office:infopath:Template1:-myXSD-2010-07-21T14-21-13\"");
xmldoc.AppendChild(xmlPi);