.Net 4.8 Projects, even with Language 9.0, run multiple analyzers

[senecarr]

When I started switching our stuff to Mapperly, I started getting this error on everything I build:
A partial method may not have multiple implementing declarations
After digging, I found that there is Mapperly Analyzer in the Analyzer "folder" of every project with it, and one for every version of Roslyn in that version of Mapperly's libraries. At build, that means every one of them is generating the partial methods. Even though they match, that's multiple implementation and the builds error out.
For the time being, the work around I found was to manually go into the project.assets.json and remove extra Roslyn dll paths.
I can see that someone else had this with 4.7 and even when trying to switch to 4.8. I'm using the older version of project, not SDK, so I think that's the common factor.
Would it be possible to change up the packaging or make a Mapperly that's for older csproj format that only includes one analyzer?

[latonz]

Thank you for reporting this. My understanding was that Roslyn automatically resolves the analyzer from the latest supported version-specific directory.
What roslyn version are you using (#error version)? Does the build also fail on the command line / in a container?

[senecarr]

I haven't tried with a container. I have tried creating a new project in 4.8 and get the same issue. Running the build from the command line seems to have the same issue.
I have the issue when trying to build with VS 2026 and VS 2022. When using VS 2026, I'm seeing 5.4.0-2 when doing #error version.

[latonz]

I’m not entirely certain about the specific .nupkg structure for this scenario and I cannot find documentation on how these source generator roslyn versions are resolved.
Unfortunately, I don't have a Windows environment available to verify the behavior firsthand. If anyone has a Windows machine handy to test things and can clarify the package structure, that would be very helpful.

[senecarr]

What I ended up having to do to get it to work was update all my projects that use it to use the newer SDK style CSproj format.