Optional

A Sub receives arguments. Sometimes a default value makes sense for a parameter. With Optional, we can specify a default value and then omit it when calling a method.

Optional arguments are a syntax improvement. The VB.NET compiler inserts the default values for an Optional argument in all calling locations.

Optional example

Let us begin with this simple example. The "name" argument here is Optional. Its default value, specified with an assignment expression, is the string "Empty."

Module Module1

    Sub PrintName(ByVal Optional name As String = "Empty")
        ' The "name" argument will have a default value if no new as specified.
        Console.WriteLine(name)
    End Sub

    Sub Main()
        ' Specify no argument.
        PrintName()
        ' Specify arguments.
        PrintName("Cat")
        PrintName("Dog")
    End Sub

End Module
Empty
Cat
Dog

Named arguments

In VB.NET we find named argument syntax. Here we combine Optional arguments and named arguments. The compiler resolves all this syntax.

Module Module1

    Sub Print(ByVal Optional name As String = "Empty",
              ByVal Optional size As Integer = 0)
        ' Use string interpolation to print name and size arguments.
        Console.WriteLine($"{name} {size}")
    End Sub

    Sub Main()
        ' Specify both arguments in order.
        Print("Cat", 2)
        ' Use 1 named argument.
        Print(name:="Bird")
        ' Use 1 named argument at last position.
        Print("Dog", size:=10)
        ' Use 2 named arguments.
        Print(size:=100, name:="Horse")
    End Sub

End Module
Cat 2
Bird 0
Dog 10
Horse 100

Some notes

We can add Optional keywords to all our methods. But this rapidly becomes confusing. Instead, using Optional only on some parameters on often-called methods is a good approach.

Optional and named arguments are syntax improvements to the VB.NET language. They simplify things when used correctly. Programmer skill is essential to using them well.