Nullness issue - null annotations on F# record members are not visible from C#

[mlaily]

Issue description

Hi!
I couldn't find a definitive source or existing issue for my problem so I'm creating this issue...

Problem:

type MyRecord = {
    NonNullableString: string
    NullableString: string | null
}

With this record, I was hoping to see that NullableString would have type string? from C#, but it doesn't (it has type string).

Taking a look at ILSpy, I see that some nullness metadata are added, but on the backing field of the property only, which is not public: (internal string? NullableString@;)

The public properties and the constructor do not appear to have nullness metadata.

Consequence:

Interop between F# and C# is inconsistent regarding nullness:

I was hoping to use this feature to declare DTOs from my F# project and expose them in a web API implemented in a C# ASP.NET Core project.
Swagger/OpenAPI with Swashbuckle already support reading nullness metadata on properties to show this info in the generated specs, but because of this issue it doesn't work.

Question:

Is that behavior on purpose?

Did I miss something in the specs somewhere?

Is there any plan to change the current behavior?

Thanks.

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

.NET 9

Reproducible code snippet and actual behavior

type MyRecord = {
    NonNullableString: string
    NullableString: string | null
}

Possible workarounds

No response

[vzarytovskii]

Yep, we need to inherit nullness attributes in this codegen.