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: 
There are several options. I've found Irony to be good. It's powerful, and it has a novel syntax utilizing C# operator overloads to convey the syntax of your grammars directly in C#.

Irony - .NET Language Implementation Kit.  

<http://irony.codeplex.com/>
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