vba - How do optional Parameters in Let/Get Properties work?

Question

I am using vba with Excel 2007, and am writing code for a class module.

1) Is the following code even possible?...
Essentially I have two enums, call them eDATASET and eDATATSUBSET. A particular value from eDATASET should trigger an assignment from the optionally passed parameter in a Let property. Something like this:

Public Property Let foo(Optional ByVal lngSubSet as eDATASUBSET, _  
                        ByVal lngSuperSet as eDATASET)
    Select Case lngSuperSet
        Case eDATASET.abc, eDATASET.def
            mlngBar = lngSuperSet
        Case eDATASET.xyz
            '// if lngSubSet not passed, trigger error code...
            mlngBar = lngSubSet
    End Select
End Property

2) How do I even pass an optional parameter to a writable property when calling the object...
Aside from the seemingly backwards placement of the Optional parameters (compared with optional parameters in functions and subs), I am having trouble finding any documentation on this feature. The vba help says this:

Optional. Indicates that an argument is not required. If used, all subsequent arguments in arglist must also be optional and declared using the Optional keyword. Note that it is not possible for the right side of a Property Let expression to be Optional.

and the following from vbusers.com. Neither explain much in the way of usage. So how would i pass the optional parameter when calling the object from a code module... oObj.foo = ???

3) Is there a better way to do this?...
I have a basic understanding of oop (at least in how it is implemented in vba). Is there a better way to conditionally accept a parameter into an object?

Answer 1

1) Yes your code is possible.

2) This is how you pass an argument:

Assuming myObject is an object of your class:

myObject.foo(lngSubSet) = lngSuperSet 

The placement of arguments in the arglist does indeed look weird, but that's VBA for you. Say you have 4 arguments, two of which are optional, plus your right hand side. You would place them like this:

Public Property Let foo(arg1, arg2, Optional arg3, Optional arg4, _  
                        RHS)

and use them like this (assuming you're opting out of arg4):

myObject.foo(arg1,arg2,arg3) = RHS

3) Is there a better way to do this? There always is, depending who you ask. You could have your lngSubSet argument as a separate property entirely. That's how I tend to do it. But in your case, your way of doing things may work well for you. I don't know, it's largely a question of taste and dependent on your specific application.