How do I write a Parser in C#?

Well... where to start with this one.... First off, writing a parser, well that's a very broad statement especially with the question your asking. Your opening statement was that you wanted a simple arithmatic "parser" , well technically that's not a parser, it's a lexical analyzer, similar to what you may use for creating a new language. ( http://en.wikipedia.org/wiki/Lexical_analysis ) I understand however exactly where the confusion of them being the same thing may come from. It's important to note, that Lexical analysis is ALSO what you'll want to understand if your going to write language/script parsers too, this is strictly not parsing because you are interpreting the instructions as opposed to making use of them. Back to the parsing question.... This is what you'll be doing if your taking a rigidly defined file structure to extract information from it. In general you really don't have to write a parser for XML / HTML, beacuse there are already a ton of them around, and more so if your parsing XML produced by the .NET run time, then you don't even need to parse, you just need to "serialise" and "de-serialise". In the interests of learning however, parsing XML (Or anything similar like html) is very straight forward in most cases. if we start with the following XML: <movies> <movie id="1"> <name>Tron</name> </movie> <movie id="2"> <name>Tron Legacy</name> </movie> <movies> we can load the data into an XElement as follows: XElement myXML = XElement.Load("mymovies.xml"); you can then get at the 'movies' root element using 'myXML.Root' MOre interesting however, you can use Linq easily to get the nested tags: var myElements = from p in myXML.Root.Elements("movie") select p; Will give you a var of XElements each containing one '...' which you can get at using somthing like: foreach(var v in myElements) { Console.WriteLine(string.Format("ID {0} = {1}",(int)v.Attributes["id"],(string)v.Element("movie")); } For anything else other than XML like data structures, then I'm afraid your going to have to start learning the art of regular expressions, a tool like "Regular Expression Coach" will help you imensly ( http://weitz.de/regex-coach/ ) or one of the more uptodate similar tools. You'll also need to become familiar with the .NET regular expression objects, ( http://www.codeproject.com/KB/dotnet/regextutorial.aspx ) should give you a good head start. Once you know how your reg-ex stuff works then in most cases it's a simple case case of reading in the files one line at a time and making sense of them using which ever method you feel comfortable with. A good free source of file formats for almost anything you can imagine can be found at ( http://www.wotsit.org/ )

C# is almost a decent functional language, so it is not such a big deal to implement something like Parsec in it. Here is one of the examples of how to do it: http://jparsec.codehaus.org/NParsec+Tutorial It is also possible to implement a combinator-based http://pdos.csail.mit.edu/~baford/packrat/thesis/, in a very similar way, but this time keeping a global parsing state somewhere instead of doing a pure functional stuff. In my (very basic and ad hoc) implementation it was reasonably fast, but of course a code generator like http://code.google.com/p/peg-sharp/ must perform better.

I know that I am a little late, but I just published a parser/grammar/AST generator library named Ve Parser. you can find it at http://veparser.codeplex.com or add to your project by typing 'Install-Package veparser' in Package Manager Console. This library is kind of Recursive Descent Parser that is intended to be easy to use and flexible. As its source is available to you, you can learn from its source codes. I hope it helps.