C# 8.0 is available for use in Visual Studio 2019 Preview. Mads Torgersen explains some of the key features in this video.
He explains how to enforce usage in a single file using
What Mads doesn't explain is how to fully accept the new Nullable Reference Types for your entire project.
The secret is to add this to your .csproj file:
It goes here:
Now your whole project enforces nullable reference types!
Credit to this Reddit comment.