It does break legacy code so you need to opt-in per file or per project. It also doesn't fix legacy code automatically. Any of your dependencies that haven't yet opted in (and thus added the right annotations to their assemblies) to strict null checking are assumed `Foo?` (as they've always been) and may still give NullReferenceExceptions.

Almost all of the BCL (base class library; the system libraries) has been annotated at this point, but there will be plenty of libraries that are not yet on NuGet still.

If you miss the question mark and you've opted in to strict null checks you won't get a null pointer exception, you'll get a compiler error when trying to assign null. (That's why you have to opt-in: it makes `Foo` without the question mark non-nullable.)