Visual Basic 2008 9.0 .NET Examples and Ebook

Parameter can be defined as optional by placing the Optional keyword before ByVal or ByRef and by assigning a defaultvalue to that paramter.

The defaultvalue is used when the call provides no parametervalue for that argument.

The second call (1) in next example will use 2 for parameter exponent.

Module Example1
    Sub Main()
        Console.WriteLine(GetPower(3, 3))
        Console.WriteLine(GetPower(3))                                     ' (1)
        '
        Console.ReadLine()
    End Sub
    Function GetPower(ByVal base As Integer, _
                      Optional ByVal exponent As Integer = 2) As Integer
        GetPower = base ^ exponent
    End Function
End Module

Output :

 27
 9

Defaultvalues are mandatory on Optional parameters, and can not be used on nonoptional parameters. The defaultvalue must be expressed by a constant expression ( for instance a literal ), that is know at compiletime.

Methods can define more than one Optional parameter. In that case all optional parameters must be places at the end of the parameterlist.

Module Example2
    Sub Main()
        ShowSum(1)       ' value1 = 1 | value2 = 2 | value3 = 3            ' (1)
        ShowSum(2, 3)    ' value1 = 2 | value2 = 3 | value3 = 3            ' (1)
        ShowSum(4, , 5)  ' value1 = 4 | value2 = 2 | value3 = 5            ' (1)
        ShowSum(6, 7, 8) ' value1 = 6 | value2 = 7 | value3 = 8            ' (1)
        '
        ShowSum(value1:=1)                                                 ' (2)
        ShowSum(value1:=2, value2:=3)                                      ' (2)
        ShowSum(value1:=4, value3:=5)                                      ' (2)
        ShowSum(value1:=6, value2:=7, value3:=8)                           ' (2)
        '
        ShowSum(value3:=5, value1:=4)                                      ' (3)
        ShowSum(value3:=8, value2:=7, value1:=6)                           ' (3)
        '
        ShowSum(4, value3:=5)                                              ' (4)
        '
        Console.ReadLine()
    End Sub
    Sub ShowSum(ByVal value1 As Integer, _
                Optional ByVal value2 As Integer = 2, _
                Optional ByVal value3 As Integer = 3)
        Console.WriteLine(value1 + value2 + value3)
    End Sub
End Module

Output :

 6
 8
 11
 21
 6
 8
 11
 21
 11
 21
 8

Use commas to indicate which parametervalues should be assigned to which parametervariables.

Since Visual Basic 9.0 ( 2008 ) argument can also be passed by name (2).

The order in which the parametervalues are provided is arbitrary when all parametervalues are passed by name (3).

A combination of argument passing by position and by name is possible (4).