The CSharp samples on Github use dotnet core 2.0. This version reached end-of-life on Oct 1, 2018 - https://github.com/dotnet/core/blob/master/microsoft-support.md. These samples should be migrated to at least dotnet core 2.1 - LTS till August 21, 2021.
Compiling currently results in compiler warnings. I submitted an issue on Github csharp examples should be upgraded to dotnet core 2.1 · Issue #208 · square/connect-api-examples (github.com) and a PR Fixes#208 Update to dotnet core 21 by c0g1t8 · Pull Request #209 · square/connect-api-examples (github.com).