Manufacture a Missing value

Manufacture a Missing value

Visual Basic doesn’t provide you with a means for creating a Missing value, a feature that in some cases would prove useful in order to simplify the syntax of calls to procedures that expects a variable number of arguments. It isn’t difficult, however, to create such a value programmatically, as follows:

' never pass an argument to this functionFunction MissingValue(Optional DontPassMe As Variant) As Variant    ' If an argument is passed, raise the "Invalid Procedure Call" error    If Not IsMissing(DontPassMe) Then Err.Raise 5           MissingValue = DontPassMeEnd Function

There is an alternative way, however, that doesn’t require that you intentially avoid to pass an Optional argument to a procedure. This latter method actually manufactures the internal representation of a Variant:

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (dest As _    Any, source As Any, ByVal bytes As Long)Function MissingValue() As Variant    ' A "missing" value is actually an error code equal to hex 80020004    ' so here we store the integer part of this value    MissingValue = &H80020004    ' and then we modify the VarType to vbError (= 10)    CopyMemory MissingValue, 10, 2End Function

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