I worked at NLP (Natural Language Processing) for an industry leader for a while, and what you want to do is not a trivial task. I know one of the creators nltk, and I myself used it; it's a high quality open source tool, and I would recommend using it (do you have a particularly good reason to use C #?)
POS marking is usually implemented by training a language model using manual annotated data, then applying this model to a new text, predicting parts of speech and providing confidence. nltkhas tools that do this, and they also have some models (if I'm not mistaken).
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