I'm trying to use C#'s XmlReader on a large series of XML files, they are all properly formatted except for a few select ones (unfortunately I'm not in a position to have them changed, because it would break a lot of other code).
The errors only come from one specific part of the these affronting XML files and it's ok to just skip them but I don't want to stop reading the rest of the XML file.
The bad parts look like this:
<InterestingStuff>
...
<ErrorsHere OptionA|Something = "false" OptionB|SomethingElse = "false"/>
<OtherInterestingStuff>
...
</OtherInterestingStuff>
</InterestingStuff>
So really if I could just ignore invalid tags, or ignore the pipe symbol then I would be ok.
Trying to use XmlReader.Skip() when I see the name "ErrorsHere" doesn't work, apparently it already reads a bit ahead and throws the exception.
TLDR: How do I skip so I can read in the XML file above, using the XmlReader?
Edit:
Some people suggested just replacing the '|'-symbol, but the idea of XmlReader is to not load the entire file but only traverse parts you want, since I'm reading directly from files I can not afford the read in entire files, replace all instances of '|' and then read parts again :).
I've experimented a bit with this in the past.
In general the input simply has to be well-formed. An XmlReader will go into an unrecoverable error-state when the basic XML rules are broken. It is easy to avoid schema-validation but that's not relevant here.
Your only option is to clean the input, that can be done in a streaming manner (custom Stream or TextReader) but that will require a light form of parsing. If you don't have pipe-symbols in valid positions it's easy.
XmlReader is strict. Any non-conformance, it will error.
So no, you can't do that unless you write your own xml implementation. Fixup on the malformed data is probably easier.
Once I had a similar situation (with HTML files, not XML files). But I ended up using regular expression for each HTML file before entering it into my operation pipeline, to delete malformed parts. It came handy and was easier than struggling with the API. :)
Related
I need to read potentially large (~300mb) XML files, and edit some of the nodes. Basically I need to:
Read the XML from the start
Whenever I find a node called trgt
Add some text to it
What's the best way to approach this in C#? Which XML classes should I use to find and edit the nodes I need to change?
TIA
VTD-XML is the only XML parsing lib that supports a feature called incremental update. It is also memory efficient and performant. But it requires you to download it as a third party lib.
From my experience of transforming some very large (2GB+) xml files (don't ask!) I found xsl transforms to be the quickest - The engines involved are heavily optimised for such tasks, compare to any manual looping etc you might try.
you can use Linq-to-XML. in short, read with XDocument, parse and add data with Linq. This will not be the fastest code, but will probably be the quickest to write.
If you have memory constraints, you will probably have to parse it manually (i.e. load only part of it in memory, process that part, replace it in the file)
If it's a fairly simple operation similar to find-and-replace, you could try treating it as a normal text file instead of an xml document. I imagine that might be faster than all the xml parsing.
Does anyone have/make/sell an error tolerant XML reader for .NET?
Yeah, I know, XML isn't designed to have errors in it and should be rejected if it's not valid .. blah blah. But sadly the real-world is imperfect and developers do make mistakes and I still want to be able to read their feeds even if I'm missing the odd element here or there because it wasn't encoded properly or had some other error in it. So please, no answers "fix the source" or "reject it".
So, does anyone have a component that can recover and handle common mistakes in XML files?
It's precisely because the real world is imperfect that XML is so widely used. What would be the functional specification for an error-tolerant XML parser? It's an open-ended problem. It's hard enough to parse all variations of well-formed XML without trying to second-guess all possible errors.
[... Waits for downvote.]
Look around HTML Parser, 'cause html is almost xml
Run the XML through Beautiful Soup first. That will clean your XML of errors so it parses correctly
For the specific case of an RSS feed and the specific case of individual corrupt item entries, you can use XmlTextReader to manually read in each item separately, handling the XmlException for invalid items. When an Exception occurs, you'll need to use a new Reader instance, as the original Reader is hosed. You'll still have to have valid <item> and </item> tags to identify each item, but you'll be able to recover from corrupt data within each item.
yes, I know it's old question, but recently I was looking for tolerant xml parser and found the following: XmlParser.
A Roslyn-inspired full-fidelity XML parser with no dependencies and a
simple Visual Studio XML language service.
The parser produces a full-fidelity syntax tree, meaning every
character of the source text is represented in the tree. The tree
covers the entire source text. The parser has no dependencies and can
easily be made portable.
You can add Nugets in your project. I tried this parser and it can read any XML files.
In the thread What’s your favorite “programmer ignorance” pet peeve?, the following answer appears, with a large amount of upvotes:
Programmers who build XML using string concatenation.
My question is, why is building XML via string concatenation (such as a StringBuilder in C#) bad?
I've done this several times in the past, as it's sometimes the quickest way for me to get from point A to point B when to comes to the data structures/objects I'm working with. So far, I have come up with a few reasons why this isn't the greatest approach, but is there something I'm overlooking? Why should this be avoided?
Probably the biggest reason I can think of is you need to escape your strings manually, and most new programmers (and even some experienced programmers) will forget this. It will work great for them when they test it, but then "randomly" their apps will fail when someone throws an & symbol in their input somewhere. Ok, I'll buy this, but it's really easy to prevent the problem (SecurityElement.Escape to name one).
When I do this, I usually omit the XML declaration (i.e. <?xml version="1.0"?>). Is this harmful?
Performance penalties? If you stick with proper string concatenation (i.e. StringBuilder), is this anything to be concerned about? Presumably, a class like XmlWriter will also need to do a bit of string manipulation...
There are more elegant ways of generating XML, such as using XmlSerializer to automatically serialize/deserialize your classes. Ok sure, I agree. C# has a ton of useful classes for this, but sometimes I don't want to make a class for something really quick, like writing out a log file or something. Is this just me being lazy? If I am doing something "real" this is my preferred approach for dealing w/ XML.
You can end up with invalid XML, but you will not find out until you parse it again - and then it is too late. I learned this the hard way.
I think readability, flexibility and scalability are important factors. Consider the following piece of Linq-to-Xml:
XDocument doc = new XDocument(new XDeclaration("1.0","UTF-8","yes"),
new XElement("products", from p in collection
select new XElement("product",
new XAttribute("guid", p.ProductId),
new XAttribute("title", p.Title),
new XAttribute("version", p.Version))));
Can you find a way to do it easier than this? I can output it to a browser, save it to a document, add attributes/elements in seconds and so on ... just by adding couple lines of code. I can do practically everything with it without much of effort.
Actually, I find the biggest problem with string concatenation is not getting it right the first time, but rather keeping it right during code maintenance. All too often, a perfectly-written piece of XML using string concat is updated to meet a new requirement, and string concat code is just too brittle.
As long as the alternatives were XML serialization and XmlDocument, I could see the simplicity argument in favor of string concat. However, ever since XDocument et. al., there is just no reason to use string concat to build XML anymore. See Sander's answer for the best way to write XML.
Another benefit of XDocument is that XML is actually a rather complex standard, and most programmers simply do not understand it. I'm currently dealing with a person who sends me "XML", complete with unquoted attribute values, missing end tags, improper case sensitivity, and incorrect escaping. But because IE accepts it (as HTML), it must be right! Sigh... Anyway, the point is that string concatenation lets you write anything, but XDocument will force standards-complying XML.
I wrote a blog entry back in 2006 moaning about XML generated by string concatenation; the simple point is that if an XML document fails to validate (encoding issues, namespace issues and so on) it is not XML and cannot be treated as such.
I have seen multiple problems with XML documents that can be directly attributed to generating XML documents by hand using string concatenation, and nearly always around the correct use of encoding.
Ask yourself this; what character set am I currently encoding my document with ('ascii7', 'ibm850', 'iso-8859-1' etc)? What will happen if I write a UTF-16 string value into an XML document that has been manually declared as 'ibm850'?
Given the richness of the XML support in .NET with XmlDocument and now especially with XDocument, there would have to be a seriously compelling argument for not using these libraries over basic string concatenation IMHO.
I think that the problem is that you aren't watching the xml file as a logical data storage thing, but as a simple textfile where you write strings.
It's obvious that those libraries do string manipulation for you, but reading/writing xml should be something similar to saving datas into a database or something logically similar
If you need trivial XML then it's fine. Its just the maintainability of string concatenation breaks down when the xml becomes larger or more complex. You pay either at development or at maintenance time. The choice is yours always - but history suggests the maintenance is always more costly and thus anything that makes it easier is worthwhile generally.
You need to escape your strings manually. That's right. But is that all? Sure, you can put the XML spec on your desk and double-check every time that you've considered every possible corner-case when you're building an XML string. Or you can use a library that encapsulates this knowledge...
Another point against using string concatenation is that the hierarchical structure of the data is not clear when reading the code. In #Sander's example of Linq-to-XML for example, it's clear to what parent element the "product" element belongs, to what element the "title" attribute applies, etc.
As you said, it's just awkward to build XML correct using string concatenation, especially now you have XML linq that allows for simple construction of an XML graph and will get namespaces, etc correct.
Obviously context and how it is being used matters, such as in the logging example string.Format can be perfectly acceptable.
But too often people ignore these alternatives when working with complex XML graphs and just use a StringBuilder.
The main reason is DRY: Don't Repeat Yourself.
If you use string concat to do XML, you will constantly be repeating the functions that keep your string as a valid XML document. All the validation would be repeated, or not present. Better to rely on a class that is written with XML validation included.
I've always found creating an XML to be more of a chore than reading in one. I've never gotten the hang of serialization - it never seems to work for my classes - and instead of spending a week trying to get it to work, I can create an XML file using strings in a mere fraction of the time and write it out.
And then I load it in using an XMLReader tree. And if the XML file doesn't read as valid, I go back and find the problem within my saving routines and corret it. But until I get a working save/load system, I refuse to perform mission-critical work until I know my tools are solid.
I guess it comes down to programmer preference. Sure, there are different ways of doing things, for sure, but for developing/testing/researching/debugging, this would be fine. However I would also clean up my code and comment it before handing it off to another programmer.
Because regardless of the fact you're using StringBuilder or XMLNodes to save/read your file, if it is all gibberish mess, nobody is going to understand how it works.
Maybe it won't ever happen, but what if your environment switches to XML 2.0 someday? Your string-concatenated XML may or may not be valid in the new environment, but XDocument will almost certainly do the right thing.
Okay, that's a reach, but especially if your not-quite-standards-compliant XML doesn't specify an XML version declaration... just saying.
How do I make the XDocument object save an attribute value of a element with single quotes?
I'm not sure that any of the formatting options for LINQ to XML allow you to specify that. Why do you need to? It's a pretty poor kind of XML handler which is going to care about it...
As long as you use single- and double-quotes in matched pairs and with correct nesting, standards-compliant XML processors won't care which style you use. Your question suggests that you are intending to process your XML output with tools that are not standards-compliant (or perhaps even not XML-aware). This is a dicey proposition at best, though I recognize that work situations and customer demands may not always give you the options of working with the right tools. I have co-workers who use sed and grep to sift through and modify XML files, and they often can get away with that. But if you have any choice at all, I recommend that you handle XML files with XML-aware tools all along the pipeline up to the point where the data is no longer marked up in XML. Doing otherwise will result in systems that are much more fragile than if you used XML-aware tools for all XML processing.
If you can't do that, then JacobE's suggestion is probably your best bet.
If it is absolutely necessary to have single quotes you could write your XML document to a string and then use a string replace to change from single to double quotes.
I have inherited a poorly written web application that seems to have errors when it tries to read in an xml document stored in the database that has an "&" in it. For example there will be a tag with the contents: "Prepaid & Charge". Is there some secret simple thing to do to have it not get an error parsing that character, or am I missing something obvious?
EDIT:
Are there any other characters that will cause this same type of parser error for not being well formed?
The problem is the xml is not well-formed. Properly generated xml would list the data like this:
Prepaid & Charge
I've fixed the same problem before, and I did it with this regex:
Regex badAmpersand = new Regex("&(?![a-zA-Z]{2,6};|#[0-9]{2,4};)");
Combine that with a string constant defined like this:
const string goodAmpersand = "&";
Now you can say badAmpersand.Replace(<your input>, goodAmpersand);
Note a simple String.Replace("&", "&") isn't good enough, since you can't know in advance for a given document whether any & characters will be coded correctly, incorrectly, or even both in the same document.
The catches here are you have to do this to your xml document before loading it into your parser, which likely means an extra pass through the document. Also, it does not account for ampersands inside of a CDATA section. Finally, it only catches ampersands, not other illegal characters like <. Update: based on the comment, I need to update the expression for hex-coded (&#x...;) entities as well.
Regarding which characters can cause problems, the actual rules are a little complex. For example, certain characters are allowed in data, but not as the first letter of an element name. And there's no simple list of illegal characters. Instead, large (non-contiguous) swaths of UNICODE are defined as legal, and anything outside that is illegal.
When it comes down to it, you have to trust your document source to have at least a certain amount of compliance and consistency. For example, I've found people are often smart enough to make sure the tags work properly and escape <, even if they don't know that & isn't allowed, hence your problem today. However, the best thing would be to get this fixed at the source.
Oh, and a note about the CDATA suggestion: I use that to make sure xml I'm creating is well-formed, but when dealing with existing xml from outside, I find the regex method easier.
The web application isn't at fault, the XML document is. Ampersands in XML should be encoded as &. Failure to do so is a syntax error.
Edit: in answer to the followup question, yes there are all kinds of similar errors. For example, unbalanced tags, unencoded less-than signs, unquoted attribute values, octets outside of the character encoding and various Unicode oddities, unrecognised entity references, and so on. In order to get any decent XML parser to consume a document, that document must be well-formed. The XML specification requires that a parser encountering a malformed document throw a fatal error.
The other answers are all correct, and I concur with their advice, but let me just add one thing:
PLEASE do not make applications that work with non well-formed XML, it just makes the rest of our lives more difficult :).
Granted, there are times when you really just don't have a choice if you have no control over the other end, but you should really have it throwing a fatal error and complaining very loudly and explicitly about what is broken when such an event occurs.
You could probably take it one step further and say "Ack! This XML is broken in these places and for these reasons, here's how I tried to fix it to make it well-formed: ...".
I'm not overly familiar with the MSXML APIs, but most good XML parsers will allow you to install error handlers so that you can trap the exact line/column number where errors are appearing along with getting the error code and message.
Your database doesn't contain XML documents. It contains some well-formed XML documents and some strings that look like XML to a human.
If it's at all possible, you should fix this - in particular, you should fix whatever process is generating the malformed XML documents. Fixing the program that reads data out of this database is just putting wallpaper over a crack in the wall.
You can replace & with &
Or you might also be able to use CDATA sections.
There are several characters which will cause XML data to be reported as badly-formed.
From w3schools:
Characters like "<" and "&" are illegal in XML elements.
The best solution for input you can't trust to be XML-compliant is to wrap it in CDATA tags, e.g.
<![CDATA[This is my wonderful & great user text]]>
Everything within the <![CDATA[ and ]]> tags is ignored by the parser.