base128 | Encoding to and decoding from base128 | Base64 library

Unless you use something like google.protobuf.Any (which includes the message name in the data), you should be fine. The name of the message isn't generally part of the data serialized in protocol buffers. (And it could be different between different languages, or in a different package etc.)

I don't know what protobuf-net does with unknown fields, but within Google.Protobuf, the deserialization process will succeed even if there are fields with tag numbers that weren't known about at generation time. (Depending on the version used, the unknown field values may or may not be retained for later serialization. I wouldn't expect protobuf-net to retain values when these are just normal user classes.) This is something you probably want to test, so that if you do add any new fields to Bar and serialize data using those fields, the Foo-only clients are still able to deserialize. (You need to think about the impact of that loss of data though.)

When you pass a Nullable into an API that takes object, it is boxed, and the boxing rules for Nullable are special: it boxes to either a regular null, or a box of T, not a box of T?; in other words: an object value never contains a nullable value-type. Because of this, from the library's perspective you sent a ConsoleColor, not a ConsoleColor?, so: