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AppendChild in a for-loop (node transfer)

I am a beginner in c#, so don't expect a lot from me...
How can I transfer nodes from one xml file to another and store it in the new xml file using c#?
The procedure I took was the following:
Load 2 xml files by using variablename.load(filepath)
Creating XmlElements which store root node of the imported xml-files.
Nested for-loop across the nodes in the xml files.
If nodes of the documents are the same, don't do anything
Else: Remove the content of the previous XML file and update it with the content of the new XML-file
Apparently implementing appendChild results in a System.NullReferenceException: error

How do I set a xml Node to contain a string's content?

Let's say I have one string called data.
How do i put a string's text inside one of it's nodes? How do i choose which one contains the string?
unlike the other questions i saw, my xml will contain many tags. so i dont want so set it to contain one thing, but to add it to the others.

You can use the following to add a text node to your XML file with the data you have:
// Open the XML
XmlDocument doc = new XmlDocument();
doc.LoadXml("<somedata><moredata>sometext</moredata></somedata>");
// Create the new node, set to text and insert the data
XmlNode newElem = doc.CreateNode("text", "yournodename", "");
newElem.InnerText = data;
// Write the new node and append
XmlElement root = doc.DocumentElement;
root.AppendChild(newElem);
If you need something more specific, please provide some more information regarding the node name or the XML Document etc.
UPDATE: As mentioned in the comments, the above code will modify but not save the modifications to the XML. To do so, load the XML document using doc.Load("pathtoyourxml.xml") instead of using doc.LoadXml() and save it to the same path using doc.Save("pathtoyourxml.xml") after doing whatever you are doing with it.

Overwrite specific XML node

I have a XML file of the following format:
<Alarms>
<Alarm>
<Id>1</Id>
<Severity>Warning</Severity>
<Comments></Comments>
</Alarm>
<Alarm>
<Id>2</Id>
<Severity>Error</Severity>
<Comments>Restart the machine</Comments>
</Alarm>
...
My program has a GUI which gives the user the ability to edit the Comments of an alarm. I am trying to come up with the best solution for the actions to take when a user is done editing and wants to save the changes. The XML file isn't extremely large (it does not warrant a database) but large enough that I do not want to overwrite the entire thing every time a change is made to a single alarm. Is it possible to target only a specific node and edit the Comments attribute without then having to re-write everything?
I'm looking for a XML-specific solution... I want to avoid regular flat-file methods that involve going to a specific line in a file and then editing that line. Perhaps something exists for XML files that I'm not privy to. I'm currently working with a .NET 2 project but will soon be upgrading to 4.5, so any solution works for me.

You can load up the xml in XmlDocument class. Navigate with an XPath query to the Comments node you want to edit and change the value. When you are done, just save the document to the same file name or a different one.
Here is an example using a Console Application.
// The Id of the Alarm to edit
int idToEdit = 2;
// The new comment for the Alarm
string newCommentValue = "Here is a new comment";
XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);
XmlNode commentsElement = doc.SelectSingleNode(String.Format("Alarms/Alarm[Id = '{0}']/Comments", idToEdit));
commentsElement.InnerText = newCommentValue;
doc.Save(Console.Out);
Here is a working fiddle: https://dotnetfiddle.net/eQROet

Copy an existing XML element into the same document

I am building up an XML document using c#. I use AppendChild to add an element called say "test" to a parent element in the document.
I then build up more of the xmlDocument but further down the same document I wish to append the same element "test" to a different node.
I have tried using AppendChild but it added it to the new node and removed it from the existing one. Which I guess is expected. I was just wondering is there anything available that I can use to copy the existing element and add it to a new node without removing it from the existing position?
To perhaps help explain the following code results in the test element only appearing in parentElement2.
parentElement1.AppendChild(test)
...
parentElement2.AppendChild(test)
Is there anyway I can insert test into both parent elements?
Hope this makes sense.

This should be possible with the CloneNode method. It lets you create a (possibly deep) copy of a node, which you can then insert wherever you like in your document.

C# LINQ to XML - How to write specific XElements to file

I was wondering how i can write a specific section of an XDocument to file.
Suppose I load the entire document when my application starts up, and then read settings using application load. When I modify a property of my class, I wish to write JUST THAT PROPERTY (or just that property and its children) back to the file, leaving unchanged any other modification to the XDocument (in memory)
My current code look like the following (note I have some wrappers around the XDocument and XElement classes):
public void SaveRecursiveData()
{
//Load the original file into a new document
XmlConfig tmp = new XmlConfig(_XmlDoc.Filename,false);
//find the node i am interested in
XElement currentElement = tmp.Xmldoc.XPathSelectElement(this.Path);
//Replace it with my IN MEMORY one
currentElement.ReplaceWith(_XmlNode);
//Write the whole temporary document back to the file
tmp.Save();
}
Is this the best approach or is there another way?

You can just do
currentElement.Attribute("toChange").Value = "mynewvalue";
instead of ReplaceWith