[feoren]

That is tedious with or without a source generator, mainly because there's a much better way to do it:

    public Optional<string> Name;

With Optional being something like:

    class Optional<T> {
      public T? Value;
      public bool IsSet;
    }

I'm actually partial to using IEnumerable for this, and I'd reverse the boolean:

    class Optional<T> {
      public IEnumerable<T> ValueOrEmpty;
      public bool IsExplicitNull;
    }

With this approach (either one) you can easily define Map (or "Select", if you choose LINQ verbiage) on Optional and go delete 80% of your "if" statements that are checking that boolean.

Why mess with source generators? They're just making it slightly easier to do this in a way that is really painful.

I'd strongly recommend that if you find yourself wanting Null to represent two different ideas, then you actually just want those two different ideas represented explicitly, e.g. with an Enum. Which you can still do with a basic wrapper like this. The user didn't say "Null", they said "Unknown" or "Not Applicable" or something. Record that.

    public OneOf<string, NotApplicable> Name

A good OneOf implementation is here (I have nothing to do with this library, I just like it):

https://github.com/mcintyre321/OneOf

I wrote a JsonConverter for OneOf and just pass those over the wire.