Various online sentiment classification services are available now. Such giants as Google, Amazon, or Microsoft offer their cloud solutions for natural language processing. But if you do not plan to pay 1-2$ for 1000 API calls (price increases when the review has more than 1000 characters) and/or are ready to prepare your local classification service or program, then you can use offline classifiers. There are several .NET classifiers for the offline case that will be discussed in this post.