Updated: August 2009
Returns a Boolean value that indicates whether an expression contains no valid data (Null).
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.
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 illustrates the use of the IsNull function.
'Dim MyVar, Result ' IsNull returns False. Result = IsNull(MyVar) ' IsNull returns True. MyVar = Null Result = IsNull(MyVar) ' IsNull returns False MyVar = Empty Result = IsNull(MyVar)
If the expression consists of more than one variable, Null in any constituent variable causes True to be returned for the entire expression.
' IsNull returns True. MyVar = Null + 4 Result = IsNull(MyVar)
Expressions that you might expect to evaluate to True under some circumstances, such as If Var = Null and If Var <> Null, are always False. This is because any expression that contains a Null is itself Null, and therefore False.
' This returns False, because an expression ' that contains Null is itself Null. MyVar = Null If MyVar = Null Then Result = True Else Result = False End If