Directive

A directive modifies programs in an early stage of compilation. The #Const directive defines a value. And the #If directive can test those constants.

Other directives, like #Region, are also available. Directives add a layer of compile-time logic to VB.NET programs. They can make programs harder to read.

Const, If

This program uses the #Const directive to set the SPECIAL symbol to the value 1. Inside the Main Sub, we use an #If directive to test the value of SPECIAL.

So When SPECIAL is equal to 1, the variable "a" is assigned to the value 2. Other values of SPECIAL do not have this effect.

Detail We use preprocessor directives, which begin with the pound character "#". These directives start at the first character in a line.

Info They are not the same as the Const and If keywords used in compiled VB.NET code.

#Const SPECIAL = 1

Module Module1

    Sub Main()
        ' Local variable.
        Dim a As Integer = 1

        ' Use special value is SPECIAL is 1.
#If SPECIAL = 1 Then
        a = 2
#End If

        ' Display value.
        Console.WriteLine(a)
    End Sub
End Module
2

Region

Another directive available in VB.NET programs is #Region. This directive partitions a code file. It has no effect on the compiled program.

Info We use #Region to organize code. In Visual Studio, a region can be collapsed and expanded.

Also Regions are used by Visual Studio when auto-generation of code files is performed, as for Windows Forms.

Note Regions in VB.NET cannot be used in all locations. They must enclose an entire Function body or other unit of code.

Module Module1

#Region "INIT"
    ''' <summary>
    ''' Initialize the program.
    ''' </summary>
    Sub Main()
        Console.WriteLine("A")
        Console.WriteLine("B")
        Console.WriteLine("C")
        Console.WriteLine("Copyright 2017")
    End Sub
#End Region

End ModuleA
B
C
Copyright 2017

A discussion

Preprocessing directives are handled at an early stage of compilation. The preprocessor scans VB.NET programs before compiling code statements into intermediate language.

Info Directives are separate from the rest of the VB.NET code. This gives them special abilities.

And With preprocessing directives, no performance loss is incurred with branches of an #If. The branches are handled before execution.

Warning One problem with preprocessing directives is bit rot. Code that is not compiled because of an #If may become buggy or invalid.

Directives add another layer of compilation to VB.NET programs. With #If, #Const and even #Region, we access another set of abilities in this language.