LDM Notes for 05/05/2025

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-05-05.md

Agenda

• Extensions
  • XML Docs
  • Participation in pattern-based constructs
  • Skeleton type metadata
  • Single-phase consequences

Feel free to read the date in either MM/DD/YYY or DD/MM/YYYY format, whichever suits your preferences.

[CyrusNajmabadi]

I prefer DM/MD/YYYY format. Thanks for catering to me.

[yaakov-h]

the fifth day of the fiftieth month?

[En3Tho]

Glad to see that Single/Two phase inference got some discussion time again.

To me it's the biggest pain point when dealing with extensions/generics.

[RenderMichael]

Will there be support for [UnsafeAccessor] with extensions? In the current preview, it doesn't work:

using System.Runtime.CompilerServices;

var item = new C();

Console.WriteLine(item.GetSecretOld()); // prints "boo"
Console.WriteLine(item.GetSecretNew()); // blows up at build time


static class PublicClassExtensions
{
    extension(C item)
    {
        // error CS0106: The modifier 'extern' is not valid for this item
        // error CS0501: 'PublicClassExtensions.extension(C).GetSecretNew()' must declare a body because it is not marked abstract, extern, or partial
        [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_secret")]
        public extern ref string GetSecretNew();
    }

    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_secret")]
    public static extern ref string GetSecretOld(this C item);
}

class C
{
    private string _secret = "boo";
}

Initially I was looking to ask if properties would support the unsafe accessor, but it looks like even the one-to-one translation currently doesn't.

[333fred]

UnsafeAccessor is a runtime feature. You'd need to ask them over in dotnet/runtime.

[RenderMichael]

Point taken.

There does seem to be some potential language work. Based on the error message, it seems like extern is not a valid modifier on an extension.

Is that preview funk, or is that intended behavior?

[333fred]

I'm not sure about extern. @jcouv?

[jjonescz]

extern is currently disallowed by the spec. See also dotnet/roslyn#78572.

[jcouv]

Adding an open issue to the spec to track the extern question. Thanks