VB .NET Language in a Nutshell

Assemblies and VB .NET

To a VB .NET programmer, an assembly is similar to a traditional DLL or EXE file, except that it contains additional information, such as reference and type information (which in COM was often contained in a separate OLB or TLB file, called a type library). When a VB .NET application is compiled, the compiler creates an assembly for the target EXE or DLL.

In the .NET environment, namespaces are part of assemblies. An assembly can contain many namespaces, and namespaces can be nested.

For instance, the System namespace is the fundamental namespace in the .NET environment. This is not the time to go into details, but one example will be useful. The System namespace identifies the Array class (Microsoft likes to say that the namespace contains classes.) One of the members of the Array class is the Copy method, which copies a portion of one array to another array. Thus, we can write code such as the following:

Imports System     ' Optional since System is always imported
Dim array1(  ) As Integer = {1, 2, 3, 4}
Dim array2(3) As Integer
Array.Copy(array1, array2, 3)

To use an existing assembly in a VB .NET project, you must do two things:

• Add a reference to the assembly to your project. There are two exceptions to this rule, however. A reference to the assembly containing the System namespace (mscorlib.dll) is added automatically, as is a reference to the assembly containing the language being used (for VB .NET, this is Microsoft.VisualBasic.dll).

• Access the member or members of the namespace, as described later in this section.

To access a member of a namespace, you can use its fully qualified name. For example, to create an instance of the Timers class, which is found in the System.Timers namespace, you can use a code fragment like the following:

Dim oTimer As New System.Timers.Timer(2000)

Since using fully qualified names tends to be relatively cumbersome, you can include an Imports statement at the beginning of a code file, before any references to variables or classes. Its syntax is:

Imports [aliasname = ] namespace

where aliasname is an optional alias for the namespace, and namespace is its fully qualified name. For example, if you import the System.Timers namespace as follows:

Imports System.Timers

you do not have to qualify a reference to the Timer class, which can be instantiated as follows:

Dim oTimer As New Timer(2000)

In the event that there is a naming conflict (either two namespaces have identically named types, or a named type conflicts with a name in your project), you can specify an alias for the namespace, as follows:

Imports TI = System.Timers

and then instantiate a Timer object as follows:

Dim oTimer As New TI.Timer(2000)

Tip

If you’re using the Visual Basic command-line compiler, you have to explicitly import the Microsoft.VisualBasic namespace, or your code will not compile. If you’re using Visual Studio, VB’s language elements are accessed automatically without your having to import the namespace.