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In Visual Basic 6, various program objects were defined by placing source code in files having various
filename extensions. For example, code that defined classes was placed in .cls files, code that defined
standard modules was placed in .bas files, and so on. In Visual Basic .NET, all source files have .vb
filename extensions, and program objects are defined with explicit syntax. For example, classes are
defined with the Class...End Class construct, and standard modules are defined with the
Module...End Module construct. Any particular .vb file can contain as many of these declarations
as desired.
The purpose of standard modules in Visual Basic 6 was to hold code that was outside of any class
definition. For example, global constants, global variables, and procedure libraries were often placed
in standard modules. Standard modules in Visual Basic .NET serve a similar purpose and can be used
in much the same way. However, in Visual Basic .NET they define datatypes that cannot be
instantiated and whose members are all static. This will be discussed in more detail in Chapter 2.
The next line in the example begins the definition of a subroutine named Main:
Public Sub Main( )
It ends with:
End Sub
This syntax is similar to Visual Basic 6. The Sub statement begins the definition of a subroutine -- a
method that has no return value.
The Main subroutine is the entry point for the application. When the Visual Basic .NET compiler is
invoked, it looks for a subroutine named Main in one of the classes or standard modules exposed by
the application. If Main is declared in a class rather than in a standard module, the subroutine must be
declared with the Shared modifier. This modifier indicates that the class does not need to be
instantiated for the subroutine to be invoked. In either case, the Main subroutine must be Public. An
example of enclosing the Main subroutine in a class rather than in a standard module is given at the
end of this section.
If no Main subroutine is found, or if more than one is found, a compiler error is generated. The
command-line compiler has a switch (/main:location) that allows you to specify which class or
standard module contains the Main subroutine that is to be used, in the case that there is more than
one.
Lastly, there's the line that does the work:
Console.WriteLine("hello, world")
This code invokes the Console class's WriteLine method, which outputs the argument to the console.
The WriteLine method is defined as a shared (also known as a static) method. Shared methods don't
require an object instance in order to be invoked; nonshared methods do. Shared methods are
invoked by qualifying them with their class name (in this case, Console).
Here is a program that uses a class instead of a standard module to house its Main subroutine. Note
that Main is declared with the Shared modifier. It is compiled and run in the same way as the
standard module example, and it produces the same output. There is no technical reason to choose
one implementation over the other.
Imports System
Public Class Hello
Public Shared Sub Main( )
Console.WriteLine("hello, world")
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