Non-nullable parameters syntax rationale

[heyajulia]

With C# 11, we finally get a syntax for non-nullable parameters (is that the right term?) that lets us do away with explicit null checks and instead lets us use a sleek syntax, Type paramName!!.

My question is about those two exclamation marks.

Why isn't it Type paramName! (one exclamation mark) or Type! paramName (which mirrors Type? paramName) or something else entirely?

[bernd5]

A non nullable parameter is simply Type paramName in nullable aware code. With Type paramName!! you get a non nullable parameter with an explicit runtime check (a potential nullable violation causes just a warning which can be ignored or suppressed).
This can be used for example for your public API which wants to enforce non null values for certain parameters. If your API might be used in a non-nullable aware environment you can avoid issues based on contract violations with this feature.

[heyajulia]

Yes, I know that. I'm just curious why that specific syntax was chosen.

[bernd5]

It is not Type! paramName because this feature does not affect the type - the type remains untouched.
Type paramName! would be an option

[heyajulia]

Right! That makes sense. I don't think there's some fundamental law of the universe that would prevent Type! from being a thing, but it would make the language somewhat inconsistent. Thank you!

[ufcpp]

https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-04-06.md#parameter-null-checking

[heyajulia]

Thanks, that was very interesting!

[RikkiGibson]

we finally get a syntax for non-nullable parameters (is that the right term?)

I would use the term null-checked parameters instead.