'type' is defined in an assembly that is not referenced

Question

I've run into this error in the past when compiling my .cs files.  I'm not exactly sure the reason.  I recently read that if you reference an assembly, say assembly A, then all public types referenced by assembly A must also be referenced by you.   Not sure if I worded it correctly.  My question is about the difference between compile time requirements and runtime requirements.

To test out this issue I created a class called EventSource in namespace Framework.Utility.Instrumentation in assembly Instrumentation.dll.  I created an consumer application called InstrumentationApp.exe which referenced Instrumentation.dll since it made use of the EventSource class.  All compiled fine and ran fine.  Then I created a new class called EventSource in namespace Framework.Instrumentation in assembly EventSource.dll.  I modified the first EventSource class in namespace Framework.Utility.Instrumentation to derive from this new EventSource class.  I reran InstrumentationApp.exe and it ran fine.  When I tried to recompile InstrumentationApp.cs specifying a reference to Instrumentation.dll it complained that Framework.Instrumentation.EventSource is defined in an assembly that is not referenced.  Based on my previous explanation (which may not be correct) regarding compile time reference requirements I can understand why I need a reference to EventSource.dll since I have a public class which is derived from a type in a different assembly so I now need to reference that assembly.  But why does the compiler need this information yet the loaded doesn't need the information?  As I said, InstrumentationApp.exe ran fine after I modified the original EventSource having it derive from the new EventSource.

Thanks,

Nick