How can ActiveSheet, ActiveWorkbook or ThisWorkbook be set as the default for an optional parameter in VBA?

Question

I am trying to set the default value of an optional parameter to Excel's Application.ActiveWorkbook property. Here is my failed attempt:

Attempted Code

Function SheetExists(ByVal sheetName As String, _
    Optional ByVal targetBook As Workbook = Application.ActiveWorkbook) As Boolean

Result

This code caused a

Constant expression required : compile error

Questions

1. What is a "Constant Expression"?
2. Is there another way to use the ActiveWorkbook property (and other properties) as default values of parameters in functions and subroutines?

Answer 1

Can you use something like this:

Public function SheetExists(ByVal sheetName As String, Optional wb As Workbook)

  If wb Is Nothing Then Set wb =  Application.ActiveWorkbook 'or ThisWorkbook

  'your code here, for example
  wb.Application.Calculation = xlAutomatic 
End Sub