CS0027: `this` cannot be used in an attribute `nameof`

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

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[severity:It bothers me. A fix would be nice]
Given a method like this:

[MemberNotNull(nameof(this.Name))]
public MyShape SetName(string name)
{
	this.Name = name;
	return this;
}
  

CS0027 will (sometimes) be emitted as an error in VS2026, with the location referring to this. as it appears in the attribute argument. Yet the compilation succeeds anyway.

CS8618 also is emitted as a false warning in this case.

Removing this. resolves both diagnostics:

[MemberNotNull(nameof(Name))]
public MyShape SetName(string name)
{
	this.Name = name;
	return this;
}
  

This looks like a bug in the compiler / language service.

Repro steps

git clone https://github.com/AArnott/Nerdbank.MessagePack.git
cd Nerdbank.MessagePack
git checkout b1e27e7a3e5954056e4f2db72d2fc1d8ce1f2053
 .\Nerdbank.MessagePack.slnx
  

Look at the error list. You may have to open MyShape.cs in order to see it populate with these two false diagnostics.

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Original Comments

Feedback Bot on 12/22/2025, 03:14 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

Feedback Bot on 1/30/2026, 09:10 PM:

This issue is currently being investigated. Our team will get back to you if either more information is needed, a workaround is available, or the issue is resolved.

[jjonescz]

This is a regression from #81628 (comment) (cc @AlekseyTs).

[jjonescz]

This is actually by design (although previously the compiler allowed that, it was a bug). The reason you are seeing the error in the IDE only is likely because your command-line compiler is older than your IDE compiler.

nameof spec says that some errors from lookup are ignored, but this access is not part of it (this is not a name reference, it's a special keyword which is allowed only in very specific contexts, but not in attributes per the language standard).

The fix is simply to remove this.:

- [MemberNotNull(nameof(this.Name))]
+ [MemberNotNull(nameof(Name))]

[CyrusNajmabadi]

This may be a significant regression. IIRC the IDE put in extra effort to support 'this' in an attribute precisely because this was allowed, and users were writing it. If this was longstanding support, it may be good to bring to ldm to decide what to do here:

Comment is slightly wrong, since it is the compiler supporting it. But it's still something that used to work, that we def know people took advantage of. So we should at least make sure the break is acceptable.

[MulleDK19]

Yay, a BREAKING change, with no news about it, breaking every project...

This should be allowed. I like to use this to make it clear I'm referring to instance members instead of static members. Now I can no longer see that in attributes..

[CyrusNajmabadi]

@jjonescz i'm not sure why this was closed. At the very least, i think the breaking change needs to be assessed and documented :)

It really is breaking people. And we definitely know people use this feature as they have explicitly asked in the past to make our features work better in the presence of this previously allowed code :)

@jcouv for breaking change documentation process.

[jjonescz]

i think the breaking change needs to be assessed and documented

That's true, let me reopen for that.

It really is breaking people.

I think we are past the point of reverting this break for .NET 10.0.200 anyway, but we will discuss this, thanks.