I'm confused as to why this is failing. From the errors, it looks like, with this change, Microsoft.IO.Redist is turned on for linux/mac, but it should make it more restrictive, not less.

Possible things I can think of that at least might fix something:
Add parentheses
Add one of: == 'true', != 'false', or == 'false'
Replace this preprocessor directive with something (static, presumably) that actually checks the OS, but I'm not sure (how) that would work.

Any thoughts?

This PR changes the behavior of the output MSBuild assemblies based on the OS of the machine that builds them. That's not what we want; we want to change the runtime behavior of the same output MSBuild assemblies based on the runtime environment, so that the .NET Framework MSBuild avoids the Microsoft.IO.Redist codepaths when running on Mono.

I guess what I'm confused by is how to make a runtime check work cleanly. Say we had:

#if MSIOREDIST
code1
#else
code2
#endif

You could convert that to:

#if MSIOREDIST
if (mono)
code2
else
call function
#else
code2
#endif

function {
code1
}

but that feels really messy, and I'm not sure how to make that work with the property—just ignore it?

Also, I'm still confused as to why this PR would fail rather than just not doing the right thing. Any thoughts on that?