.NET Assembly References Compile Time Errors

I have assembly A with class Z that inherits from class X in assembly B. Now in a completely different solution, I have assembly C, which uses class Z.

The compiler complains unless assembly C has a reference to both assembly A & B. Even though assembly C does not use class Z directly in anyway.

Is this expected?

It seems to me that if assembly B is missing at run time stuff blows up, but at compile time it shouldn't care.

What am I missing here?

My goal is that I can tell my clients to depend on class Z in assembly A, but I can completely reconfigure my assemblies on the other side and have no effect at all on the client when they upgrade.