Guidance around CS9057

[timcassell]

We recently introduced a new analyzer in BenchmarkDotNet, however we quickly got user reports of error CS9057
dotnet/BenchmarkDotNet#2863. It's not unusual for a user to use an older sdk with BenchmarkDotNet. Can we get some guidance on how to handle it properly?

[333fred]

If you want to support people using older SDKs with BenchmarkDotNet, then you will need to change the version of Roslyn you use for your analyzer to the minimum version supported by the sdk you want to support.

[timcassell]

What if we want to support older sdks, but also want to be able to analyze newer C# features (e.g. collection expressions)? Can we somehow create 2 versions of the analyzer and load them based on the sdk version? Or it's not possible?

[333fred]

Yes, you can do that. dotnet/sdk#20793 has the full details, but the tldr is put analyzers into paths like analyzers/dotnet/roslyn3.7/cs for Roslyn 3.7, or analyzers/dotnet/roslyn4.14/cs for version 4.14.

[timcassell]

Awesome! It looks like that change was made in sdk 6.0. What will be the behavior if someone uses sdk 5.0?

[timcassell]

Should we do something like
analyzers/dotnet/roslyn4.14/cs for newer sdks, and then analyzers/dotnet/cs for older sdks?

[333fred]

What will be the behavior if someone uses sdk 5.0?

Both of those SDKs are well and truly out of support, so hopefully the answer is "we don't need to care" 🙂

[333fred]

Should we do something like
analyzers/dotnet/roslyn4.14/cs for newer sdks, and then analyzers/dotnet/cs for older sdks?

Just put everything into their respective folders. Don't try to do defaults.

[RikkiGibson]

(I was beaten to it.. but maybe saying the same thing in a slightly different way is still helpful..?)

This warning occurs when the analyzer was compiled against too new of a version of Roslyn for the host it was loaded into. When this happens, the analyzer could start calling new APIs which don't exist in that old host's version of Roslyn and things will fail at runtime.

Your analyzer project should reference a version of Roslyn which is old enough to represent a supported API surface in the .NET SDKs, IDE versions, etc. you intend for your analyzer to be compatible with.

https://learn.microsoft.com/en-us/dotnet/core/porting/versioning-sdk-msbuild-vs#supported-net-versions
https://learn.microsoft.com/en-us/visualstudio/extensibility/roslyn-version-support?view=visualstudio