Excel's Formula - sisman

Chapter 25: VBA Custom Function Examples 709
Custom functions that you develop in VBA can also have optional arguments. You specify an
optional argument by preceding the argument’s name with the keyword Optional. The following
is a simple function that returns the user’s name:
Function USER()
USER = Application.UserName
End Function
Suppose that in some cases, you want the user’s name to be returned in uppercase letters. The
following function uses an optional argument:
Function USER(Optional UpperCase As Variant) As String
If IsMissing(UpperCase) Then UpperCase = False
If UpperCase = True Then
USER = Ucase(Application.UserName)
Else
USER = Application.UserName
End If
End Function
If you need to determine whether an optional argument was passed to a function,
you must declare the optional argument as a variant data type. Then you can use the
IsMissing function within the procedure, as demonstrated in this example.
If the argument is FALSE or omitted, the user’s name is returned without any changes. If the
argument is TRUE, the user’s name converts to uppercase (using the VBA Ucase function)
before it is returned. Notice that the first statement in the procedure uses the VBA IsMissing
function to determine whether the argument was supplied. If the argument is missing, the statement
sets the UpperCase variable to FALSE (the default value).
Optional arguments also allow you to specify a default value in the declaration, rather than testing
it with the IsMissing function. The preceding function can be rewritten in this alternate
syntax as
Function USER(Optional UpperCase As Boolean = False) As String
If UpperCase = True Then
USER = UCase(Application.UserName)
Else
USER = Application.UserName
End If
End Function