Const

A Const variable cannot be changed. It is immutable. In the VB.NET language, Strings (and other types) can be decorated with the Const keyword.

Value types such as Integer may also be made constant. This influences performance. Const values resolve faster than regular variables. They should be used when possible.

This program uses a Const at the level of the Module: the syntax here is similar to a field. It also uses a Const at the level of the method: the syntax resembles a local variable Dim.

Module Module1

    ''' <summary>
    ''' Const can be Module or Class level.
    ''' </summary>
    Const _x As Integer = 1000

    Sub Main()
        ' Const can be Function or Sub level.
        Const name As String = "Sam"

        ' Used as an argument.
        Console.WriteLine(name)
        Console.WriteLine(_x)

        ' Constant cannot be the target of an assignment.
        '_x = 2000
    End Sub

End Module
Sam
1000

Performance

Fields and variables—declared with Dim—designate memory locations. When a Dim variable is evaluated, the execution engine reads from that memory location.

Const values help clear up confusing code. With Const we construct self-documenting code. A Const with the identifier "name" is known to be a name by any programmer reading the code.