Telegram.Bot is the most popular .NET Client for Telegram Bot API. The Bot API is an HTTP-based interface created for developers keen on building bots for Telegram. Check Bots: An introduction for developers to understand what a Telegram bot is and what it can do. All Bot API methods are already documented by Telegram but this book covers all you need to know to create a chatbot in .NET. There are also many concrete examples written in C#. The guides here can even be useful to bot developers...
...This form based AI developed in c# is a recreation of my previous console based application. development has now stoped on that project but can still be found at http://www.openpoppy.info and at https://sourceforge.net/projects/artificialintelligence/
Teknik is an attempt at a natural language 'chatbot'. When interfaced with other projects like Festival and Sphinx, Teknik could be used as a true personal assistant.