The expression argument can be any expression.
IsNull returns True if expression is Null, that is, it contains no valid data; otherwise, IsNull returns False. If expression consists of more than one variable, Null in any constituent variable causes True to be returned for the entire expression.
The Null value indicates that the variable contains no valid data. Null is not the same as Empty, which indicates that a variable has not yet been initialized. It is also not the same as a zero-length string (""), which is sometimes referred to as a null string.
The following example uses the IsNull function to determine whether a variable contains a Null:
Dim MyVar, MyCheck MyCheck = IsNull(MyVar) ' Returns False. MyVar = Null ' Assign Null. MyCheck = IsNull(MyVar) ' Returns True. MyVar = Empty ' Assign Empty. MyCheck = IsNull(MyVar) ' Returns False.