Splitting an Assembly Across Multiple Files

The 2005 version of C# introduced the concept of friend assemblies; the feature was added to Visual Basic in 2008. It enables you to combine assemblies in terms of what constitutes internal access. That is, you can define internal members but have them be accessible by external assemblies. This capability is useful if you intend to split an assembly across physical files but still want those assemblies to be accessible to one another as if they were internal.

You use the attribute class InternalsVisibleToAttribute to mark an assembly as exposing its internal members as friends to another assembly. This attribute is applied at the assembly level. You pass the name and the public key token of the external assembly to the attribute. The compiler then links these two assemblies as friends. The assembly containing InternalsVisibleToAttribute exposes its internals to the other assembly (and not vice versa). You can accomplish the same thing by using the command-line compiler switches.

Friend assemblies, like most things, come at a cost. If you define an assembly as a friend of another assembly, the two assemblies become coupled and need to coexist to be useful. That is, they are no longer a single unit of functionality. This can cause confusion and increase management of your assemblies. It is often easier to stay away from this feature unless you have a specific need.