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XML: how to pre-parse when only SOME data is escaped?

XML snippet:
<field>& is escaped</field>
<field>"also escaped"</field>
<field>is & "not" escaped</field>
<field>is " and is not & escaped</field>
I'm looking for suggestions on how I could go about pre-parsing any XML to escape everything not escaped prior to running the XML through a parser?
I do not have control over the XML being passed to me, they likely won't fix it anytime soon, and I have to find a way to parse it.
The primary issue I'm running into is that running the XML as is into a parser, such as (below) will throw an exception due to the XML being bad due to some of it not being escaped properly
string xml = "<field>& is not escaped</field>";
XmlReader.Create(new StringReader(xml))

I'd suggest you use a Regex to replace un-escaped ampersands with their entity equivalent.
This question is helpful as it gives you a Regex to find these rogue ampersands:
&(?!(?:apos|quot|[gl]t|amp);|#)
And you can see that it matches the correct text in this demo. You can use this in a simple replace operation:
var escXml = Regex.Replace(xml, "&(?!(?:apos|quot|[gl]t|amp);|#)", "&");
And then you'll be able to parse your XML.

Preprocess the textual data (not really XML) with HTML Tidy with quote-ampersand set to true.

If you want to parse something that isn't XML, you first need to decide exactly what this language is and what you intend to do with it: when you've written a grammar for the non-XML language that you intend to process, you can then decide whether it's possible to handle it by preprocessing or whether you need a full-blown parser.
For example, if you only need to handle an unescaped "&" that's followed by a space, and if you don't care about what happens inside comments and CDATA sections, then it's a fairly easy problem. If you don't want to corrupt the contents of comments or CDATA, or if you need to handle things like when there's no definition of &npsp;, then life starts to become rather more difficult.
Of course, you and your supplier could save yourselves a great deal of time and expense if you wrote software that conformed to standards. That's what standards are for.

Loading and reading XML file fails on its processing instruction

I'm stuck on a subtle problem. I try to build a C# 4.0 console application to read an XML file with.
The XML file is as follow:
<?xml version="1.0" encoding="UTF-8"?>
<?xml:stylesheet type='text/xsl' href='report.xsl' version='1.0'?>
...
<logs>
...
</logs>
And this is my code:
...
var root = XDocument.Load(xmlStream);
IEnumerable<XElement> address =
from el in root.Descendants("formated-text")
select el;
...
This gives me the following error at the Load method:
The ':' character, hexadecimal value 0x3A, cannot be included in a name. Line 2, position 6.
Changing the colon on the second line to a '-' solves the error ... duh
What can I do in my code to read the source XML without having to replace that 'stupid' colon first?
Thank you!

It looks to me like you simply have an invalid XML document. The colon should be a hyphen (as per W3C). I doubt that you'll be able to make LINQ to XML parse an invalid document - and you shouldn't try. You should fix the document instead.

The colon is wrong, you should be using the dash
See http://www.w3.org/Style/styling-XML.en.html

Nothing. That "stupid colon" is simply invalid at that position.

You XSL-Stylesheet element is incorrect.
It should be:
<?xml-stylesheet type='text/xsl' href='report.xsl' version='1.0'?>
Try validating your XML against any number of online validators.
You can try loading the XML as a string and fixing this issue using string parsing, or you could read the original file line by line and fix any occurences of xml:stylsheet before saving it like the text file in this example, but it would be better to get whomever created the XML to fix it at source.

I found out that the origin of these 'malformed' XML files dates back to the mid 1990's ... yes, such an old system is today still in use and still produces this output. I can live with a workaround in my code.
Thank you for taking time to provide some usefull clues at what was/is going on with these XML elements.
I needed this confirmation that the creator of the source XML did a mistake with that colon.
I already have implemented a plan B, until I can convince a really big department (not mine) to make the change in their application ... :o(
Plan B is to read the XML file first and replace all 'xml:' occurences. Then feed this corrected file into my process.

Clean out/replace invalid XML characters in element attributes

UPDATE: The invalid characters are actually in the attributes instead of the elements, this will prevent me from using the CDATA solution as suggested below.
In my application I receive the following XML as a string. There are a two problems with this why this isn't accepted as valid XML.
Hope anyone has a solution for fixing these bug gracefully.
There are ASCII characters in the XML that aren't allowed. Not only the one displayed in the example but I would like to replace all the ASCII code with their corresponding characters.
Within an element the '<' exists - I would like to remove all these entire 'inner elements' (<L CODE="C01">WWW.cars.com</L>) from the XML.
<?xml version="1.0" encoding="ISO-8859-1"?>
<cars>
<car model="ford" description="Argentinië love this"/>
<car model="kia" description="a small family car"/>
<car model="opel" description="great car <L CODE="C01">WWW.cars.com</L>"/>
</cars>

For a quick fix, you could load this not-XML into a string, and add [CDATA][1] markers inside any XML tags that you know usually tend to contain invalid data. For example, if you only ever see bad data inside <description> tags, you could do:
var soCalledXml = ...;
var xml = soCalledXml
.Replace("<description>", "<description><![CDATA[")
.Replace("</description>", "]]></description>");
This would turn the tag into this:
<description><![CDATA[great car <L CODE="C01">WWW.cars.com</L>]]></description>
which you could then process successfully -- it would be a <description> tag that contains the simple string great car <L CODE="C01">WWW.cars.com</L>.
If the <description> tag could ever have any attributes, then this kind of string replacement would be fraught with problems. But if you can count on the open tag to always be exactly the string <description> with no attributes and no extra whitespace inside the tag, and if you can count on the close tag to always be </description> with no whitespace before the >, then this should get you by until you can convince whoever is producing your crap input that they need to produce well-formed XML.
Update
Since the malformed data is inside an attribute, CDATA won't work. But you could use a regular expression to find everything inside those quote characters, and then do string manipulation to properly escape the <s and >s. They're at least escaping embedded quotes, so a regex to go from " to " would work.
Keep in mind that it's generally a bad idea to use regexes on XML. Of course, what you're getting isn't actually XML, but it's still hard to get right for all the same reasons. So expect this to be brittle -- it'll work for your sample input, but it may break when they send you the next file, especially if they don't escape & properly. Your best bet is still to convince them to give you well-formed XML.
using System.Text.RegularExpressions;
var soCalledXml = ...;
var xml = Regex.Replace(soCalledXml, "description=\"[^\"]*\"",
match => match.Value.Replace("<", "<").Replace(">", ">"));

You could wrap that content in a CDATA section.
With regex it will be something like this, match
"<description>(.*?)</description>"
and replace with
"<description><![CDATA[$1]]></description>"

Alternatives to XDocument

Hey guys, XDocument is being very finicky with one of the xml feeds I have to parse, and keeps giving me the error
'=' is an unexpected token. The expected token is ';'. Line 1, position 576.
Which is basically XDocument crying about a loose "=" sign in the XML document.
I don't have any control over the source XML document, so I need to either get XDocument to ignore this error, or use some other class. Any ideas on either one?

If the document isn't well-formed XML (and my guess is that you have '&=' in the document or some other entity-looking string) then it's unlikely that any other XML parsers are going to be any happier with it. Have you tried loading the document in, say, IE to see if it parses there or pasted to an XML validator? You can also just try XmlDocument.Load() and see if it parses there, that's the next closest XML parser (aside from XmlReader which takes a little bit of setting up).

It won't make for good XML, but if you need to just load up a bad document then the HTML Agility Pack is a good tool. It can overlook many of the things that make HTML not XHTML and not XML-like, so your erroneous XML input will likely be parsed too. The object model it expresses is similar to XmlDocument. e.g.
HtmlDocument doc = new HtmlDocument();
doc.Load("file.xml");
foreach(HtmlNode link in doc.DocumentElement.SelectNodes("//a[#href"])
{
HtmlAttribute att = link["href"];
att.Value = FixLink(att);
}
doc.Save("file.htm");
Or you can use Agility Pack to clean up the XML and then feed its clean output to a real XML parser for further processing.
This is a quick and dirty trick that I've used for one-time tasks. It's not necessarily recommended over a proper solution.
What I would recommended if time permits is to somehow format/fix the erroneous XML content (e.g. maybe in its string form, or using another tool) before feeding it to an XML parser.

Take a look at the answers of this question: Parsing an XML/XHTML document but ignoring errors in C#
The best option I believe is to parse it in a try/catch block, remove the offending block inside the catch block, and re-parse.

XmlDocument dropping encoded characters

My C# application loads XML documents using the following code:
XmlDocument doc = new XmlDocument();
doc.Load(path);
Some of these documents contain encoded characters, for example:
<xsl:text>
</xsl:text>
I notice that when these documents are loaded,
gets dropped.
My question: How can I preserve <xsl:text>
</xsl:text>?
FYI - The XML declaration used for these documents:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

Are you sure the character is dropped? character 10 is just a line feed- it wouldn't exactly show up in your debugger window. It could also be treated as whitespace. Have you tried playing with the whitespace settings on your xmldocument?
If you need to preserve the encoding you only have two choices: a CDATA section or reading as plain text rather than Xml. I suspect you have absolutely 0 control over the documents that come into the system, therefore eliminating the CDATA option.
Plain-text rather than Xml is probably distasteful as well, but it's all you have left. If you need to do validation or other processing you could first load and verify the xml, and then concatenate your files using simple file streams as a separate step. Again: not ideal, but it's all that's left.

is a linefeed - i.e. whitespace. The XML parser will load it in as a linefeed, and thereafter ignore the fact that it was originally encoded. The encoding is just part of the serialization of the data to text format - it's not part of the data itself.
Now, XML sometimes ignores whitespace and sometimes doesn't, depending on context, API etc. As Joel says you may find that it's not missing at all - or you may find that using it with an API which allows you to preserve whitespace fixes the problem. I wouldn't be at all surprised to see it turned into an unencoded linefeed character when you output the data though.

maybe it would be better to keep data in ![CDATA] ?
http://www.w3schools.com/XML/xml_cdata.asp