Reference to Window Object in VBA

Question

I have a problem referencing a windows object in VBA. it throws the following error: "Error 5 (Invalid procedure call or argument). I cannot find the cause, because I see no programming error.

Public Sub TestWindowhandle()

Dim lResult As Long
Dim objShell, wins, winn
Dim IE_Count As Long, i As Long, This_PID As Long


On Error GoTo TestWindowhandle_Error

Set objShell = CreateObject("Shell.Application")
Set wins = objShell.Windows
IE_Count = wins.Count
For i = 0 To (IE_Count - 1)
   Set winn = wins.Item(i)
Next i

On Error GoTo 0
Exit Sub

TestWindowhandle_Error:

MsgBox "Error " & Err.Number & " (" & Err.Description & ") in line " & Erl & " in procedure TestWindowhandle of Module Module1"
Stop
End Sub

Answer 1

Something odd with that interface, it seems to only work with a copy of the control variable so:

Set winn = wins.Item(i + 0)

or

Set winn = wins.Item((i))

Answer 2

I believe here is what is happening.

The Item method accepts a Variant parameter.

When calling an external method that accepts a Variant parameter, VB likes to create and pass Variants that provide the value by reference - that is, with the VT_BYREF flag set.
However VB does not set this flag when sending out intermediate results (temporaries, not stored in a variable), which makes sense because even if the called method updates the value, no one will be able to see it.

So when you call .Item(i), VB sends out a Variant of type VT_I4 | VT_BYREF, and when you call .Item(i + 0), a Variant of type VT_I4 is dispatched, without the VT_BYREF.

In most situations the difference is not significant because VARIANT-aware methods should be able to cope with either. However this particular method does different things depending on exactly which VT_ it receives, and because of this it is explicitly willing to reject any VT_s other than the three accepted ones. So in order to call it you need to trick VB into removing the byref flag.

Interestingly, when you declare the variable as Variant, VB still dispatches a VT_VARIANT | VT_BYREF, but the method seems to support this situation and correctly resolves to the pointed-to inner Variant that has the non-reference type of VT_I4.

Note this has nothing to do with VB's ByVal/ByRef - this is about the internal structure of the VARIANT data type.