Optional default-value & IsMissing(argument)

[ReneMiner]

IsMissing is Keyword working for pb and vb that simply tells if an optional parameter was passed or not.
The byref way works at least for strings alike

...opt byref sData as string,...

If VarPtr(sData) then
' there is a strptr!
ergo IsMissing(sData) will be False

I am not sure if the optional default parameter-values were fixed in the meantime - i did not use them for years now.

And an interesting way that is certainly more useful then Function_CParams which sometimes does not return the correct count of passed arguments that is implemented to vb6 - (the syntax reminds of AutoIt/AutoHotKey/Relax and others using ":=" which were probably also an option for cHash to let remain empty buckets instead of confusing the colon with the End-Of-Line symbol
(btw. help wrongly mentions "semicolon" ( multiple times there where it should be "colon"(

I just paste the vb6-way here
- of course it were nonsense to copy the need of keyword OPTIONAL for every optional parameter

Using Optional Arguments

You can specify arguments to a procedure as optional by placing the Optional keyword in the argument list. If you specify an optional argument, all subsequent arguments in the argument list must also be optional and declared with the Optional keyword. The two pieces of sample code below assume there is a form with a command button and list box.
For example, this code provides all optional arguments:
Dim strName As String
Dim strAddress As String

Sub ListText(Optional x As String, Optional y _
As String)
List1.AddItem x
List1.AddItem y
End Sub

Private Sub Command1_Click()
strName = "yourname"
strAddress = 12345 ' Both arguments are provided.
Call ListText(strName, strAddress)
End Sub
This code, however, does not provide all optional arguments:
Dim strName As String
Dim varAddress As Variant

Sub ListText(x As String, Optional y As Variant)
List1.AddItem x
If Not IsMissing(y) Then
List1.AddItem y
End If
End Sub

Private Sub Command1_Click()
strName = "yourname" ' Second argument is not
' provided.
Call ListText(strName)
End Sub
In the case where an optional argument is not provided, the argument is actually assigned as a variant with the value of Empty. The example above shows how to test for missing optional arguments using the IsMissing function.
Providing a Default for an Optional Argument

It's also possible to specify a default value for an optional argument. The following example returns a default value if the optional argument isn't passed to the function procedure:
Sub ListText(x As String, Optional y As _
Integer = 12345)
List1.AddItem x
List1.AddItem y
End Sub

Private Sub Command1_Click()
strName = "yourname" ' Second argument is not
' provided.
Call ListText(strName) ' Adds "yourname" and
' "12345".
End Sub
...

Creating Simpler Statements with Named Arguments

For many built-in functions, statements, and methods, Visual Basic provides the option of using named arguments as a shortcut for typing argument values. With named arguments, you can provide any or all of the arguments, in any order, by assigning a value to the named argument.
You do this by typing the argument name plus a colon followed by an equal sign and the value

MyArgument:= "SomeValue"

and placing that assignment in any sequence delimited by commas.
Notice that the arguments in the following example are in the reverse order of the expected arguments:

Function ListText(strName As String, Optional strAddress As String)
List1.AddItem strName
List2.AddItem strAddress
End Sub

Private Sub Command1_Click()
ListText strAddress:=”12345”, strName:="Your Name"
End Sub

This is especially useful if your procedures have several optional arguments that you do not always need to specify.