[tialaramex]

I think sum types in general and Option<T> in particular is nicer. But the reason C# has nullability isn't that they disagree with me, it's that fundamentally the CLR has the same model as Java, all these types can be null, even though in the modern C# language you can say "No, not null that's never OK" at runtime on the CLR too bad maybe it's null.

For example if I write a C# function which takes a Goose, specifically a Goose, not a Goose? or similar - well, too bad the CLR says my C# function can be called by this obsolete BASIC code which has no idea what a Goose is, but it's OK because it passed null. If my code can't cope with a null? Too bad, runtime exception.

In real C# apps written by an in-house team this isn't an issue, Ollie may not be the world's best programmer but he's not going to figure out how to explicity call this API with a null, he's going to be stopped by the C# compiler diagnostic saying it needs a Goose, and worst case he says "Hey tialaramex, why do I need a Goose?". But if you make stuff that's used by people you've never met it can be an issue.

[dwattttt]

> For example if I write a C# function which takes a Goose, specifically a Goose, not a Goose? or similar - well, too bad the CLR says my C# function can be called by this obsolete BASIC code which has no idea what a Goose is, but it's OK because it passed null. If my code can't cope with a null? Too bad, runtime exception.

That's actually no different to Rust still; if you try, you can pass a 0 value to a function that only accepts a reference (i.e. a non-zero pointer), be it by unsafe, or by assembly, or whatever.

Disagreeing with another comment on this thread, this isn't a matter of judgement around "who's bug is it? Should the callee check for null, or the caller?". Rust's win is by clearly articulating that the API takes non-zero, so the caller is buggy.

As you mention it can still be an issue, but there should be no uncertainty around who's mistake it is.

[int_19h]

The difference is that C# has well-defined behavior in this case - a non-nullable notification is really "not-nullable-ish", and there are cases even in the language itself where code without any casts in it will observe null values of such types. It's just a type system hole they allow for convenience and back-compat.

OTOH with Rust you'd have to violate its safety guarantees, which if I understand correctly triggers UB.

[steveklabnik]

> which if I understand correctly triggers UB.

Yes, your parent's example would be UB, and require unsafe.