Nullable annotation at the file level/deliberate mis-annotating functions

[jhudsoncedaron]

I had previously wondered at why there's no #nullable annotations to go with <Nullable>annotations</Nullable>. We have had #nullable enable slowly spreading through the codebase but the exercise of setting <Nullable>annotations</Nullable> is still far beyond reach. I had this one finally appear in the codebase:

   string ValueOrDefault(string key, Func<string> default);

It really wants to be

   string? ValueOrDefault(string key, Func<string?> default);
   string ValueOrDefault(string key, Func<string?> default);

but that's impossible. The next best idea I had was:

   #nullable annotate
   string ValueOrDefault(string key, Func<string?> default);

That is, deliberately mis-annotating it so we don't have to deal with the fact it can't be declared; but then its own implementation can't be #nullable enable.

I've made this as a discussion item because these are rather brute force ideas and there's probably something better out there.

[333fred]

#nullable enable annotations - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#nullable-context