"Nethereum" is the .NET integration library which aims to simplify Ethereum node access and Smart Contract interaction. It can be used on both, public and permissioned Ethereum nodes running e.g. Geth, Parity or Quorum. You can find guides, examples and in-depth explanations in the official documentation.
Protip: Head over to the playground to try different Nethereum examples interactively right from within your browser. No setup required.