Section 17.3. Security Boundary

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 type definitions can't cross assemblies. You can, of course, refer to types across assembly boundaries by adding a reference to the required assembly, either in the IDE or on the command line, at compile time. What you can't do is have the definition of a type span two assemblies.

The internal access modifier limits access (for a method, for example) to the current assembly.