ByVal 关键字

方法参数（或没有关键字以 ByVal 被默认假定的）之前 ByVal 关键字说，参数将在某种程度上被发送不允许改变（设定新的值）的变量参数的基础的方法。
如果它是一个类，它不会阻止参数的内容（或状态）被更改。

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