Thanks for working on these changes. They all look good.

in Visual Studio 17.8.4, the build process fails at sandbox/SharedData.

I'm not seeing the failure in that project. But I do see a build failure in MessagePack.GeneratedCode.Tests when it runs from visual studio (CLI is fine, as you say). I'll look into this and loop in the roslyn team if I suspect a compiler bug.

One link of the chain that explains why build in VS fails is that the GeneratedMessagePackResolver type in Sandbox and SharedData is being generated as internal instead of public, making it inaccessible to the MessagePack.GeneratedCode.Tests project.

A MessagePackAnalyzer.json file for each of these projects specify that this resolver should be produced publicly. And indeed, dotnet build honors this. But the VS build doesn't.
I'll keep investigating.

I found the problem. Our in-solution .props file for activating the source generator is missing a few dependencies, causing the in-VS build to fail to deserialize the json file, and when it fails to do that, it just swallows it and proceeds with default behavior. Not great on multiple levels. I'm preparing a fix now.

Thank you very much for preparing these changes!