Visual Basic .NET Language
Palabras clave ByVal y ByRef

Palabra clave byVal

La palabra clave ByVal antes del parámetro del método (o ninguna palabra clave como ByVal se supone de forma predeterminada) dice que el parámetro se enviará de una manera que no permita que el método cambie (asigne un nuevo valor) la variable subyacente al parámetro.
No impide que se modifique el contenido (o el estado) del argumento si se trata de una clase.

Class SomeClass
    Public Property Member As Integer
End Class

Module Program
    Sub Main()
        Dim someInstance As New SomeClass With {.Member = 42}

        Foo (someInstance)
        ' here someInstance is not Nothing (still the same object)
        ' but someInstance.Member is -42 (internal state can still be changed)

        Dim number As Integer = 42
        Foo(number)
        ' here number is still 42
    End Sub

    Sub Foo(ByVal arg As SomeClass)
        arg.Member = -arg.Member ' change argument content
        arg = Nothing ' change (re-assign) argument
    End Sub    

    Sub Foo(arg As Integer) ' No ByVal or ByRef keyword, ByVal is assumed
        arg = -arg
    End Sub
End Module

ByRef palabra clave

La palabra clave ByRef antes del parámetro del método dice que el parámetro se enviará de manera que permita al método cambiar (asignar un nuevo valor) la variable subyacente al parámetro.

Class SomeClass
    Public Property Member As Integer
End Class

Module Program
    Sub Main()
        Dim someInstance As New SomeClass With {.Member = 42}
    
        Foo (someInstance)
        ' here someInstance is not Nothing
        ' but someInstance.Member is -42

        Bar(someInstance)
        ' here someInstance is Nothing
    End Sub

    Sub Foo(ByVal arg As SomeClass)
        arg.Member = -arg.Member ' change argument content
        arg = Nothing ' change (re-assign) argument
    End Sub

    Sub Bar(ByRef param As Integer)
        arg.Member = -arg.Member ' change argument content
        arg = Nothing ' change (re-assign) argument
    End Sub
End Module