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Simply put, a namespace is a grouping of semantically related types contained in an assembly.

For example, the System.IO namespace contains file I/O–related types, the System.Data namespace

defines basic database types, and so on. It is very important to point out that a single assembly

(such as mscorlib.dll) can contain any number of namespaces, each of which can contain any

number of types.

To clarify, Figure 1-5 shows a screen shot of the Visual Studio 2008 Object Brower utility. This

tool allows you to examine the assemblies referenced by your current project, the namespaces

within a particular assembly, the types within a given namespace, and the members of a specific

type. Note that mscorlib.dll contains many different namespaces, each with its own semantically

related types.

Figure 1-5.

A single assembly can have any number of namespaces.

The key difference between this approach and a language-specific library such as MFC is that

any language targeting the .NET runtime makes use of the 

same namespaces and same types. For

example, the following three programs all illustrate the ubiquitous “Hello World” application, writ-

ten in C#, VB .NET, and C++/CLI: