Are you interested in joining a public blockchain? Stratis platform is the only stable native .NET Core and C# project our there. It's open source and available on Github. However, if you want to learn how to write your own blockchain, that requires learning of network protocols and network programming, cryptography, and more. Here is a basic introduction to the simplest blockchain example to get started, https://www.c-sharpcorner.com/article/blockchain-basics-building-a-blockchain-in-net-core/.
