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Visual Basic .NET - ByVal and ByRef keywords
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  ByRef keyword

ByRef keyword before method parameter says that parameter will be sent in a way allowing the method to change (assign a new value) the variable underlying the parameter.

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
  ByVal keyword

ByVal keyword before method parameter (or no keyword as ByVal is assumed by default) says that parameter will be sent in a way not allowing the method to change (assign a new value) the variable underlying the parameter.
It doesn't prevent the content (or state) of the argument to be changed if it's a class.

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



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