C# in Depth

196 CHAPTER 7 Concluding C# 2: the final featuresMost of listing 7.7 is just setting up the situation—the three lines within Main are theinteresting ones. The first line prints “Chapter7.Configuration” as the compilerresolves Configuration to that type before moving out to the namespace root. Thesecond line indicates that the type has to be in the global namespace, and so simplyprints “Configuration.” I included the third line to demonstrate that using the globalalias you can still refer to types within namespaces, but you have to specify the fullyqualified name.At this point we can get to any uniquely named type, using the global namespacealias if necessary—and indeed if you ever write a code generator where the codedoesn’t need to be readable, you may wish to use this feature liberally to make surethat you always refer to the correct type whatever other types are actually present bythe time the code is compiled. What do we do if the type’s name isn’t unique evenwhen we include its namespace? The plot thickens…7.4.3 Extern aliasesAt the start of this section, I referred to human names as examples of namespaces andcontexts. I specifically said that you’re unlikely to know more than one person calledJon Skeet. However, I know that there is more than one person with my name, and it’snot beyond the realm of possibility to suppose that you might know two or more of us.In this case, in order to specify which one you mean you have to provide some moreinformation beyond just the full name—the reason you know the particular person,or the country he lives in, or something similarly distinctive.C# 2 lets you specify that extra information in the form of an extern alias—a namethat exists not only in your source code, but also in the parameters you pass to the compiler.For the Microsoft C# compiler, this means specifying the assembly that containsthe types in question. Let’s suppose that two assemblies First.dll and Second.dllboth contained a type called Demo.Example. We couldn’t just use the fully qualifiedname to distinguish them, as they’ve both got the same fully qualified name. Instead, wecan use extern aliases to specify which we mean. Listing 7.8 shows an example of the C#code involved, along with the command line needed to compile it.Listing 7.8Working with different types of the same type in different assemblies// Compile with// csc Test.cs /r:FirstAlias=First.dll /r:SecondAlias=Second.dllextern alias FirstAlias;extern alias SecondAlias;using System;Specifies twoextern aliasesusing FD = FirstAlias::Demo;Refers to extern aliasclass TestB with namespace alias{static void Main(){Console.WriteLine(typeof(FD.Example));CUsesnamespacealiasLicensed to Rhona Hadida