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However, always remember that the using keyword is simply a shorthand notation for specify-

ing a type’s fully qualified name, and either approach results in the 

exact same underlying CIL

(given the fact that CIL code always makes use of fully qualified names) and has no effect on per-

formance or the size of the assembly.

Referencing External Assemblies

In addition to specifying a namespace via the C# using keyword, you also need to tell the C# com-

piler the name of the assembly containing the actual CIL definition for the referenced type. As

mentioned, many core .NET namespaces live within mscorlib.dll. However, the System.Drawing.

Bitmap type is contained within a separate assembly named System.Drawing.dll. A vast majority of

the .NET Framework assemblies are located under a specific directory termed the 

global assembly

cache (GAC). On a Windows machine, this can be located by default under C:\Windows\Assembly,

as shown in Figure 1-6.

Figure 1-6.

The base class libraries reside in the GAC.

Depending on the development tool you are using to build your .NET applications, you will

have various ways to inform the compiler which assemblies you wish to include during the compi-

lation cycle. You’ll examine how to do so in the next chapter, so I’ll hold off on the details for now.