The C# compiler defaults to the latest major version of the language that has been released. You may choose to compile any project using a new point release of the language. Choosing a newer version of the language enables your project to make use of the latest language features. In other scenarios, you may need to validate that a project compiles cleanly when using an older version of the language.
This capability decouples the decision to install new versions of the SDK and tools in your development environment from the decision to incorporate new language features in a project. You can install the latest SDK and tools on your build machine. Each project can be configured to use a specific version of the language for its build
Screenshot shows me selecting C# 7.2 for a .net core 2.1 application by changing the advanced options of the project properties build pane