The C# team have, for now, chosen to do the opposite of marking a variable as non-nullable; all reference types will become non-nullable by default, and you can mark the type of a variable as ‘null-safe’ by decorating them with ?, similar to nullable value types. Using a non-nullable variable that might be null (because you didn’t check if it was yet) will result in a warning, as will assigning the value of a nullable variable to one that is non-nullable.

This is really exciting