Question: I'm looking for something like flex/bison that works with C# code. I want to be able to provide some language descriptors and a parser should be generated.

Answer: 
[ANTLR](http://www.antlr.org/) is a very common parser generator with output bindings for many languages (including c#), but it is LLR which might make it a little more cumbersome than flex/bison if that's what you are accustomed to.
I've had my best luck with ANTLR, it can generate C# code as well. Good Language design support as well with ANTLR Works, an IDE for designing and testing your grammars, and quite a few tutorials to explain the whole thing.

Other option is the F# powerpack that has fsLex and fsYacc. Only brielfy poked at it, seems powerful but very little documentation. 

For C# that's the two first thing to look at i think.

GJ