Proposal: NotNullWhenArgumentIsNonZero attribute

[NN---]

Background and Motivation

public static string? PtrToStringAuto(IntPtr ptr)
        {
            return PtrToStringUni(ptr);
        }

PtrToStructure methods should be annotated as returning non null when pointer is not null

A managed string that holds a copy of the unmanaged string if the value of the ptr parameter is not null; otherwise, this method returns null.

https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.marshal.ptrtostringauto?view=netcore-3.1

Proposal:
Introduce some attribute like:

[NotNullWhenArgumentIsNonZero]
public static string? PtrToStringAuto(IntPtr ptr)
        {
            return PtrToStringUni(ptr);
        }

[NN---]

Perhaps general NotNullWhenNotNull(“ptr”)] is even better

[333fred]

We talked about additional overloads to NotNullWhen/MaybeNullWhen for error codes during initial release, but didn't see high enough usage to warrant. However, we did want to see if the community wanted them. @jcouv, I believe you've been tracking these requests?

Perhaps general NotNullWhenNotNull(“ptr”)] is even better

What's the difference between that and NotNullIfNotNull?

[NN---]

Ah, right there is such one and on the other hand it doesn’t cover this case.

For IntPtr? there are 3 value options , not 2.

[alrz]

What's the difference between that and NotNullIfNotNull?

I think that's to say NotNullIfNotDefault because IntPtr is a struct.

[333fred]

I think that's to say NotNullIfNotDefault because IntPtr is a struct.

Ah, well that's really another big feature request the current flow analysis isn't set up to track. We track null vs not null, not whether a value type is default or not.

[alrz]

FYI there's a proposal for that: #146 (looks like it's been rejected)

[Joe4evr]

@alrz It's not quite rejected as it is considered to be done as part of #99.