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Count how many differences in 2 XML files

Imagine one XML as:
<foo>
<node1>Some value</node1>
<node2>BB</node2>
<node3>TTTTT</node3>
<node4>XXXX</node4>
</foo>
and another XML as:
<foo>
<node1>Something Else</node1>
<node4>XXXX</node4>
<node5>TTTTT</node5>
</foo>
The difference count here is 3
a) node1 value is different
b) node2 is missing in 2nd XML
c) node5 is missing in 1st XML
I've tried using the XMLDiff class but the result is too cumbersome for my needs.
Schema:
Root named "foo" and one single set of childrens with one value each.
Question:
What's the simplest and fastest way to code this in C#?

One way to do this might be to generate a list of XPath assertions from your first document, in the form:
/foo/node1 = "Some value"
/foo/node2 = "BB"
/foo/node3 = "TTTT"
/foo/node4 = "XXXX"
and then apply these assertions to the second document to count how many of them are true. Because this won't catch data that is absent on the first document and present in the second, you might want to do the inverse as well. It's not perfect, of course, for example it won't catch differences in element order. But you haven't actually defined what you mean by a significant difference, and you could adjust the XPath expressions to assert what you consider significant. For example you could vary the last assertion to:
count(/foo/node4[. = "XXXX"]) = 1
The simplest and fastest way to code this, of course, is not in C#, unless that happens to be the only programming language you know. Using XSLT or XQuery would be much better.

Have you considered using XNode.DeepEquals, with the root (in this case 'foo') of each XML file being your node? The MSDN page on how to use it is here:
http://msdn.microsoft.com/en-us/library/system.xml.linq.xnode.deepequals.aspx

Xdocument with Xpath

using XDocument with xpath to parse XML
which one is better in perfomance ?
e.g. To search for tag and get value in xml
tags = xmlDoc.Descendants(xmlTag);
or
xml.SelectSingleNode("//root/node")
So which one will be faster?

XPath doesn't parse XML... it's a query language used on top of any other XML API which supports it. (For example, you can use XPath over XmlDocument or XDocument.)
To find out which query would be faster usefully, you should try your actual XPath and LINQ to XML query on samples of your actual data. I would expect XPath to be faster in some situations, and LINQ to XML to be faster in others.
However, I'd be surprised if the query execution speed was actually the bottleneck in your code - do you have evidence that it is? You should first be asking yourself which is most readable. Implement that code, then see whether it's fast enough.

replacing substring inside attributes of XmlDocument

I'm using C# with .net 3.5 and have a few cases where I want to replace some substrings in the XML attributes of an XmlDocument with something else.
One case is to replace the single quote character with ' and the other is to clean up some files that contain valid XML but the attributes' values are no longer appropriate (say replace anything attribute which starts with "myMachine" with "newMachine").
Is there a simple way to do this, or do I need to go through each attribute of every node (recursively)?

One way to approach it is to select a list of the correct elements using Linq to XML, and then iterate over that list. Here's an example one-liner:
XDocument doc = XDocument.Load(path);
doc.XPathSelectElements("//element[#attribute-name = 'myMachine']").ToList().ForEach(x => x.SetAttributeValue("attribute-name", "newMachine"));
You could also do a more traditional iteration.

I suggest taking a look at LINQ to XML. There's a collection of code snippets that can help you get started here - LINQ To XML Tutorials with Examples
LINQ to XML should allow you to do what you're looking to do, and you'll probably find it easy once you've played with it a bit.

Put prefix on all elements of xml document

I'm using C# and i need to create a XML document. Ok, i did, but, in each element i need to put a tc prefix.
The only way that i know, is using xmlDoc.CreateElement("tc", "node1", "file.xsd"), but it is very massive because i have lots of tags and my program its already writted.
Is this the only way?

This might work for you:
XmlReader - I need to edit an element and produce a new one

If you're lucky enough to be using C# 3.5, take a look at LINQ to XML.
Here's a document on How to: Create a Document with Namespaces (C#) (LINQ to XML) from MSDN for the LINQ to XML API.
And if you've never seen LINQ to XML before, take a look at this 5 minute overview

Compare two XmlNodes in C#

Is there a simple way to compare two XML structures to determine if they have the same structure and data?
I have a function that returns an XmlNode and I am trying to write unit tests for it. I store the correct XML result in a file. Durring the test I load the file into an XmlDocument, locate the proper XmlNode and compare against the result of the function. A straight compare does not work (as expected) and InnerXml does not work either.
I am considering removing all whitespace from InnerXml and comparing that, or writing my own compare to walk the tree, but I don't like either option much.

XNode.DeepEquals. Read the caveats before using it.

If you must use XmlDocument and its supporting types, consider using Microsoft's XmlDiffPatch, which performs customizable diff-operations on XML data structures.

Like CodeToGlory answered, XNode.DeepEquals() might fit your bill, check the remarks section on the MSDN page.
If you are stuck with XmlDocument (instead of XDocument), the answer is: No, there is no simple (existing way) to do it. XmlNode does not override Equals(), or provide an alternative. But it is not impossible to write, and that same Remarks section can be used as a starting point for a tree-walk algorithm.
Do get a clear picture of your requirements first, concerning Attributes, comments, CDATA nodes etc.