Returns a Boolean value indicating whether an optional Variant argument has been passed to a procedure.

Syntax

IsMissing(argname)

The required argname argument contains the name of an optional Variant procedure argument.

Remarks

Use the IsMissing function to detect whether or not optional Variant arguments have been provided
in calling a procedure. IsMissing returns True if no value has been passed for the specified argument;
otherwise, it returns False. If IsMissing returns True for an argument, use of the missing argument in
other code may cause a user-defined error. If IsMissing is used on a ParamArray argument, it always
returns False. To detect an empty ParamArray, test to see if the array’s upper bound is less than
its lower bound.

Note   IsMissing does not work on simple data types (such as Integer or Double) because, unlike
Variants, they don't have a provision for a "missing" flag bit. Because of this, the syntax for typed
optional arguments allows you to specify a default value. If the argument is omitted when the
procedure is called, then the argument will have this default value, as in the example below:

Sub MySub(Optional MyVar As String = "specialvalue")
    If MyVar = "specialvalue" Then
        ' MyVar was omitted.
    Else
    ...
End Sub

In many cases you can omit the If MyVar test entirely by making the default value equal to the value
you want MyVar to contain if the user omits it from the function call. This makes your code more
concise and efficient.

sub test(optional ai as long)

msgbox ismissing(ai)

end sub