Each C# developer knows the "internal" access level keyword — The "internal" keyword is an access modifier for types and type members. Internal types or members are accessible only within the same assembly.
But don't be fully confident in it when dealing with microsofties. Those artful guys invented in the .Net Framework 2.0 a special attribute — "InternalsVisibleToAttribute" and now your internal members can become - guess what? - public! (of course they will be visible only for specified assemblies).
No one knows the real reason for such an "invention" but this innovation has been highly estimated by test-driven development adopters because the usage of this attribute allows your test libraries to access internal classes and methods for additional testing and coverage.
I think that any other usage of it can be considered a lack of architectural design but what about the microsofties? Do they use it for product-level assemblies or for testing purposes only? Let's see. Oren Eini made some interesting investigation on how the attribute is used inside the .Net Framework:
As for Xtensive products, unit testing is the only application of the "InternalsVisibleTo" attribute.