CS0103 Triggers for nameof-expression inside extension property

[Abrynos]

Version Used: dotnet 10.0.100

Steps to Reproduce:

Create any extension property and use nameof() inside the getter or setter.

public static class Test {
  extension(string str)
  {
    public void SomeMethod() => Console.WriteLine(nameof(SomeMethod)); // this works

    public bool SomeProp
    {
      get => Console.WriteLine(nameof(SomeProp) + str); // this does not
      set => Console.WriteLine(nameof(SomeProp) + value); // this does not
    }

    public string SomeOtherProp => nameof(get_SomeOtherProp); // this works for some reason

    public string OneMoreProp => nameof(OneMoreProp); // again this does not
  }
}

Diagnostic Id: CS0103 - "The name 'SomeProp' does not exist in the current context"

Expected Behavior: Compilation succeeds

Actual Behavior: Compilation fails

[Abrynos]

The fact that the nameof(get_SomeOtherProp)-part works is especially interesting since with a normal property it is not supposed to work.

[jcouv]

This is by-design. One cannot refer to extension members without qualification. The reason nameof(SomeMethod) works is it's picking up the implementation method E.SomeMethod.
Similarly, SomeMethod(...) would bind to E.SomeMethod not E.extension(string).SomeMethod. The same applies in CREFs.

From the spec: "As we disallow unqualified references to extension members, cref would also disallow them."

[CyrusNajmabadi]

FWIW, i find this deeply unsatisfying. Especially because you can't do nameof(str.ExtProp) either. So there's like no way to reference this name safely in code, which is what nameof exists for. I'm not sure this limitation was well understood when making teh other restrictions :)

[jcouv]

This was understood by LDM, we discussed this exact scenario, but didn't have a satisfactory solution.
See dotnet/csharplang#9655 and https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-06-11.md#nameof

I'm tracking this as part of C# 15 wave of extension work as a candidate to brainstorm further.

[jcouv]

Especially because you can't do nameof(str.ExtProp) either.

See linked discussion, you can do nameof(string.ExtProp). I stand corrected, we ended up blocking this too...