Make connection between return nullable value and output nullable parameter.

[NN---]

Today you can specify parameter => return value direction:

[return: NotNullIfNotNull("output")] string? GetString(int x, out int? output)

It means if output is not null, then the return value is not null.
However, it is impossible to specify the other direction: if return value is not null, then output is not null.

I suggest adding another attribute for this: NotNullIfReturnNotNull

string? GetString(int x, [NotNullIfReturnNotNull] out int? output)

[333fred]

2 things:

1. We haven't really encountered any scenarios that would need this. Can you give a concrete example?
2. This type of flow analysis is significantly more complicated than the other direction, as the first is an input flowing to an output. This is an output flowing to a related output that can only be checked at a later point in time.

[NN---]

I have encountered it in my code when starting using nullable .

I have a method which returns not null when it succeeds and sets some output parameter.
On failure it returns null and output parameter is null.

There some workarounds like making everything output and return Boolean.
Or return an object which aggregates both using either named type or tuple.

Any workaround means rewriting code which is not a good option right now since it involves too much changes.

[333fred]

Any workaround means rewriting code which is not a good option right now since it involves too much changes.

We understand that, which is why there are lots of escape hatches built into nrt. However, I'm just pointing out that after 7 months or so, this is the first request that I'm aware of for this type of attribute, and that nullable analysis as it's built today is not equipped to handle it.

[NN---]

Unfortunately I have been busy with writing features, tests and improving stability of application for these 7 months.

I should have been started porting to nullable much earlier and open this request since long time ago;)

[333fred]

I should have been started porting to nullable much earlier and open this request since long time ago;)

No worries. In terms of feasibility, though, this request compared to your other open requests is geotagging pictures vs determining whether it's of a bird: https://xkcd.com/1425/

[spydacarnage]

As your function either effectively returns two nulls or two not-nulls, could it instead return either null or a single not-null tuple (and ignore the out variable)?

…

On Sat, 30 May 2020 at 15:46, Fred Silberberg ***@***.***> wrote: I should have been started porting to nullable much earlier and open this request since long time ago;) No worries. In terms of feasibility, though, this request compared to your other open requests is geotagging pictures vs determining whether it's of a bird: https://xkcd.com/1425/ — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <https://github.com/dotnet/csharplang/issues/3516#issuecomment-636340558>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ADIEDQM74SJ2QLGSPMNNXVTRUEL3DANCNFSM4NOUVMBQ> .

[NN---]

As I said it is a workaround and requires code change.
I suspect that I have no choice, otherwise there are too many null assertions ( variable! ) in the code.