Let Property passing argument

[tiredofit]

I seem to be confused with regard to ByRef and ByVal.

Here, regardless of ByRef or ByVal, the result in the immediate window is 100. I though ByVal DOESN'T change things.

Option Explicit


    Dim pabc As Long


Sub Test()


    abc = 10
    
    Debug.Print pabc
    
End Sub


Property Let abc(ByVal a)


    a = 100
    
    pabc = a
    
End Property

 

[tiredofit]

Yes, exactly. That's how you can tell it is being passed ByVal no matter what you specify.

But why would Msgbox x show anything other than xxx?

It's never been changed or have I missed something?

 

[RoryA]

When you call this line:

aThing.Smith = x

it calls this:

Property Let Smith(ByRef a As String)
    a = "some other string"

where x is passed as the a variable. If x were passed ByRef, then the second line there would alter it.

 

[tiredofit]

Sorry but I'm a bit thick on this one.

The Let Property expects an arg to be passed into it.

Property Let Smith(ByRef a As String)

We pass it xxx.

Then it is immediately changed in the next line to "someotherstring":

a = "some other string"

The following line assigns msmith to "someotherstring":

msSmith = a

so that when you retrieve it via the Get Property, it returns "someotherstring", as expected.

But where does x come into all this?

It's never changed, so the messagebox shows xxx, as expected.

 

[RoryA]

We pass it xxx.

No, we pass it x which contains "xxx".

 

[tiredofit]

OK, I think I get your point.

We pass it x and this line ought to change it (because we're passing ByRef):

a = "someotherstring"

but doesn't.

Thanks

 

[mikerickson]

Forget about properties for the moment.

Let's look at two subs, both take one argument and add 1 to it. But one passes it ByVal and the other ByRef

Sub One(ByVal Arg as Double)
    Arg = Arg + 1
    MsgBox "one: " & Arg
End Sub

Sub Other(ByRef Arg as Double)
    Arg = Arg +1
    MsgBox "other: " & Arg
End Sub

Lets look at two testing subs

Sub testOne()
   Dim x as Double
   x = 4.5

   MsgBox "before: " & x
   Call One (x)
   MsgBox "after: " & x
End Sub

Sub testOther()
   Dim x as Double
   x = 4.5

   MsgBox "before: " & x
   Call Other (x)
   MsgBox "after: " & x
End Sub

testOne (which uses ByVal) returns "start: 4.5" "one: 5.5" "after: 4.5"

testOther returns "start: 4.5" "other: 5.5" "after: 5.5"

When you pass an argument ByRef, anything that the called procedure does to the argument is also done to the variable in the calling routine.
"ByRef" doesn't pass the number 4.5, it passes "the variable x" so the first line of Other adds 1 to it.
"ByVal" doesn't pass the variable x, it calculates what x is at the moment and passes that value. (4.5 in this example)

That's the difference between ByVal and ByRef.

 

[confusion123]

Forget about properties for the moment.

Let's look at two subs, both take one argument and add 1 to it. But one passes it ByVal and the other ByRef

Sub One(ByVal Arg as Double)
    Arg = Arg + 1
    MsgBox "one: " & Arg
End Sub

Sub Other(ByRef Arg as Double)
    Arg = Arg +1
    MsgBox "other: " & Arg
End Sub

Lets look at two testing subs

Sub testOne()
   Dim x as Double
   x = 4.5

   MsgBox "before: " & x
   Call One (x)
   MsgBox "after: " & x
End Sub

Sub testOther()
   Dim x as Double
   x = 4.5

   MsgBox "before: " & x
   Call Other (x)
   MsgBox "after: " & x
End Sub

testOne (which uses ByVal) returns "start: 4.5" "one: 5.5" "after: 4.5"

testOther returns "start: 4.5" "other: 5.5" "after: 5.5"

When you pass an argument ByRef, anything that the called procedure does to the argument is also done to the variable in the calling routine.
"ByRef" doesn't pass the number 4.5, it passes "the variable x" so the first line of Other adds 1 to it.
"ByVal" doesn't pass the variable x, it calculates what x is at the moment and passes that value. (4.5 in this example)

That's the difference between ByVal and ByRef.

Sure, that's the textbook definition of ByRef and ByVal but as RoryA stated, when you throw Properties (Get, Let and Set) into the equation, Let and Set don't conform to the rule.

 

[RoryA]

We pass it x and this line ought to change it (because we're passing ByRef):

a = "someotherstring"

but doesn't.

Yes, that's exactly it.

It's not just that Property Let/Set default to using ByVal, but that they won't use anything else no matter what you specify. Mind you, I can't think of a good reason why a property Let/Set procedure should be able to affect the input value, so it shouldn't be a big deal.

Edit: On further testing, it is only the argument that you pass on the right hand side of the = that is always passed ByVal. If you have other arguments in a property Let/Set, they are passed as normal either ByRef or ByVal. So I've learned something new today!

 

[tiredofit]

VBA seems to have a lot of exceptions, eg transposing arrays greater than 255 elements, passing args with () and without (), etc.