I think your best bet is going to be GPLEX/GPPG, it's the closest thing to Yacc/Lex for C# that I know of, and you will need to port your actions into C# regardless.
I have also used Coco/R, ANTLR (of course), and have more recently played with Irony.net, fslex/fsyacc (F#), and fparsec (F#).
Here are some links
Fparsec
Coco/R
Irony.net
Gardens Point Parser Generator
Gardens Point Lex
I don't have a technical reason for using one versus another: I play around with these mostly for fun. I did create some DSLs for work projects a good number of years ago, but I hand rolled the scanners/parsers on those (back then I was working mostly in Pascal, and I found that TP Lex/Yacc did not suit my tastes, and the DSLs were simple enough). I have found that FParsec and Irony suit my tastes the best, as I find the other somewhat "messy" (lacking in elegance).
