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Assemblies Promote Code Reuse

As you have been building your Console Applications over the previous chapters, it may have

seemed that 

all of the applications’ functionality was contained within the executable assembly

you were constructing. In reality, your applications were leveraging numerous types contained

within the always accessible .NET code library, mscorlib.dll (recall that the C# compiler references

mscorlib.dll automatically), and in the case of some examples, System.Windows.Forms.dll.

As you may know, a 

code library (also termed a class library) is a *.dll that contains types

intended to be used by external applications. When you are creating executable assemblies, you will

no doubt be leveraging numerous system-supplied and custom code libraries as you create the

application at hand. Do be aware, however, that a code library need not take a *.dll file extension.

It is perfectly possible for an executable assembly to make use of types defined within an external

executable file. In this light, a referenced *.exe can also be considered a “code library.”

Regardless of how a code library is packaged, the .NET platform allows you to reuse types in a

language-independent manner. For example, you could create a code library in C# and reuse that

library in any other .NET programming language. It is possible to not only allocate types across lan-

guages, but also derive from them. A base class defined in C# could be extended by a class authored

in Visual Basic. Interfaces defined in Pascal .NET can be implemented by structures defined in C#,

and so forth. The point is that when you begin to break apart a single monolithic executable into

numerous .NET assemblies, you achieve a 

language-neutral form of code reuse.