17.3 Security Boundary

Assemblies form security boundaries as well as type boundaries. That is, an assembly is the scope boundary for the types it contains, and types cannot cross assemblies. You can, of course, refer to types across assembly boundaries by adding a reference to the required assembly, either in the Integrated Development Environment (IDE) or on the command line, at compile time. What you cannot do is have the definition of a type span two assemblies.