How to setup ASMDEF so it doesn't cause compile errors when NUnit (Test Runner) is not available?

I have several testing assemblies (ASMDEF) in a shared code utilities package and I want to set it up so that it still compiles even when certain Unity features (modules, packages) are disabled. If I remove the Test Runner package and all other packages I'm left with an error, because the assembly reference for nunit.framework.dll can no longer be found. Ideally, I'd like to simply use a "Define Constraint" on the ASMDEF to not compile my test assembly if the DLL is not found. How can I do this?

The following does not work:


When I remove the Test Runner package, the custom assembly is still compiled:


Alternatively, I wondered, if I could separate the dependency on NUnit from the TestRunner. Currently not sure, but it might be that I'm not using much of the test runner's features, so I could try to only reference the custom Unity NUnit directly. Would that be possible?

 

Well, as these things go, I found my answer by experimenting right after I posted my question:



It is possible to use a Version Define in the Define Constraints fields. That was unintuitive for me because I would have guessed that the assembly would first be compiled to generate the version defines, but apparently it works fine. I can now exclude testing assemblies if the Nunit extension wasn't found.