C# in Depth

Exposing internal members to selected assemblies201Listing 7.11 uses fixed-size buffers for the table of colors. Before fixed-size buffers, wecould still have used the API either with a field for each color table entry or by marshalinga normal array as UnmanagedType.ByValArray. However, this would have createda separate array on the heap instead of keeping the information all within thestructure. That’s not a problem here, but in some high-performance situations it’snice to be able to keep “lumps” of data together. On a different performance note, ifthe buffer is part of a data structure on the managed heap, you have to pin it beforeaccessing it. If you do this a lot, it can significantly affect the garbage collector. Stackbasedstructures don’t have this problem, of course.I’m not going to claim that fixed-size buffers are a hugely important feature inC# 2—at least, they’re not important to most people. I’ve included them for completeness,however, and doubtless someone, somewhere will find them invaluable.Our final feature can barely be called a C# 2 language feature at all—but it just aboutcounts, so I’ve included it for completeness.7.7 Exposing internal members to selected assembliesThere are some features that are obviously in the language—iterator blocks, for example.There are some features that obviously belong to the runtime, such as JIT compileroptimizations. There are some that clearly sit in both camps, like generics. Thislast feature has a toe in each but is sufficiently odd that it doesn’t merit a mention ineither specification. In addition, it uses a term that has different meanings in C++ andVB.NET—adding a third meaning to the mix. To be fair, all the terms are used in thecontext of access permissions, but they have different effects.7.7.1 Friend assemblies in the simple caseIn .NET 1.1 it was entirely accurate to say that something defined to be internal(whether a type, a method, a property, a variable, or an event) could only beaccessed within the same assembly in which it was declared. 3 In .NET 2.0 that’s stillmostly true, but there’s a new attribute to let you bend the rules slightly: Internals-VisibleToAttribute, usually referred to as just InternalsVisibleTo. (When applyingan attribute whose name ends with Attribute, the C# compiler will apply thesuffix automatically.)InternalsVisibleTo can only be applied to an assembly (not a specific member),and you can apply it multiple times to the same assembly. We will call the assemblycontaining the attribute the source assembly, although this is entirely unofficial terminology.When you apply the attribute, you have to specify another assembly, known asthe friend assembly. The result is that the friend assembly can see all the internal membersof the source assembly as if they were public. This may sound alarming, but it canbe useful, as we’ll see in a minute.Listing 7.12 shows this with three classes in three different assemblies.3Using reflection when running with suitable permissions doesn’t count.Licensed to Rhona Hadida