SharpNLP
The best tool for natural language processing implemented in c# is SharpNLP.
SharpNLP is a C# port of the Java OpenNLP tools, plus additional code to facilitate natural language processing.
Embedding IronPython and NLTK
Python provides a package NLTK (Natural Language Toolkit) used widely by many computational linguists, NLP researchers.
One can try to embed IronPython under C# and run NLTK from there.
You can check the following link on how to do it.