Visual Basic 2008 9.0 .NET Examples and Ebook

Just like variables, constants are "dataholders". They can be used to store data that is needed at runtime.

In contrast to variable, the content of a constant can't change at runtime, it has a constant value.
At compiletime the value for a constant must be known. The declaration line of the constant must contain an initialization clause ( where a constant expression ( for instance a literal ) is used to represent the initial value ).

Module Example1
    Sub Main()
        Const pi As Double = 3.1415
        '
        Dim radius As Single = 10
        '
        Dim circumference As Double = radius * 2 * pi
        Dim area As Double = radius ^ 2 * pi
        '
        Console.WriteLine("Circle Circumference : " & circumference)
        Console.WriteLine("Circle Area          : " & area)
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Circle Circumference : 62,83
 Circle Area          : 314,15

The above example declares a constant pi. We could have used a variable for pi, but because this value will never change during runtime, a constant was more appropriate. We protect ourself form accidentally changing this value.

It has to be said that a constant like 'pi' doesn't have to be declared. The FCL ( "Framework Class Library" ) already contains a constant for this value ( 'Math.PI' ).

Often constants are used to have an easy-to-remember name to refer to a difficult-to-remember value.

Module Example2
    Sub Main()
        Const red As Integer = &HFF0000
        Const green As Integer = &HFF00
        Const blue As Integer = &HFF
        '
        Console.WriteLine("red    : " & red)
        Console.WriteLine("green  : " & green)
        Console.WriteLine("blue   : " & blue)
        '
        Const purple As Integer = red + blue
        '
        Console.WriteLine("purple : " & purple)
        '
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 red    : 16711680
 green  : 65280
 blue   : 255
 purple : 16711935