Is it possible to mark a property of a parameter as null/not null after a function returns?

Question

If I have a function bool Foo(string? input), then I can annotate it to indicate that if the return is true then input is not null:

public static bool Foo([NotNullWhen(true)] string? input)

Is it possible to do that (or something similar) for a property of an argument? I'm thinking something along the lines of

public class AClass { string? NullableString {get;set;}}
public static bool Foo([PropertyNotNullWhen(true, nameof(AClass.NullableString))] AClass input)

I don't think MemberNotNullWhen will work in this case, because it only applies to methonds, properties, and indexers and not the arguments to them.

Hardener

This isn't possible at the moment. I would recommend opening a discussion on github.com/dotnet/csharplang/discussions/new with your real world example. I think this is definitely something that they will consider for a future language version if there's enough need. – Caseous

@YairHalberstadt Looks like that is indeed the answer - I'll give you the tick if you post it as one. I opened an issue over at github.com/dotnet/roslyn/issues/49640 , if you're interested. – Hardener

Answer 1

It seems that C# added support for this:

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/nullable-analysis

In your case you need NotNullWhen

Attribute	Category	Meaning
AllowNull	Precondition	A non-nullable parameter, field, or property may be null.
DisallowNull	Precondition	A nullable parameter, field, or property should never be null.
MaybeNull	Postcondition	A non-nullable parameter, field, property, or return value may be null.
NotNull	Postcondition	A nullable parameter, field, property, or return value will never be null.
MaybeNullWhen	Conditional postcondition	A non-nullable argument may be null when the method returns the specified bool value.
NotNullWhen	Conditional postcondition	A nullable argument won't be null when the method returns the specified bool value.
NotNullIfNotNull	Conditional postcondition	A return value, property, or argument isn't null if the argument for the specified parameter isn't null.
MemberNotNull	Method and property helper methods	The listed member won't be null when the method returns.
MemberNotNullWhen	Method and property helper methods	The listed member won't be null when the method returns the specified bool value.
DoesNotReturn	Unreachable code	A method or property never returns. In other words, it always throws an exception.
DoesNotReturnIf	Unreachable code	This method or property never returns if the associated bool parameter has the specified value.

Quahog

NotNullWhen does not solve the posed problem. It handles the case of specifying input will be non-null, but not input.NullableString. – Volga