See Also Example
Returns a value that indicates whether or not a Variant contains the special Null value.
The argument variant can be any Variant expression. The IsNull function returns True if the expression contains the Null value; otherwise, it returns False.
The Null value indicates that the Variant contains no data. Null is not the same as Empty , which indicates that a Variant has not yet been initialized. It is also not the same as a zero-length string, which is often referred to as a null string.
Caution Using the IsNull function is the only way from within Visual Basic to determine whether or not a Variant expression contains a Null value. Expressions that you might expect to evaluate True under some circumstances, such as If Var = Null and If Var <> Null, are always False. This is because any expression containing a Null is itself Null and therefore False.
IsNull Function Example
The example evaluates TestVar to determine whether it contains the Null value and then displays an appropriate message. The variable TestVar starts as Null but is changed to a zero-length string the first time through the loop. The second time through the loop, TestVar again becomes Null and the loop exits. To try this example, paste the code into the Declarations section of a form. Then press F5 and click the form.
Sub Form_Click ()
Dim TestVar ' Declare variable.
TestVar = Null ' Initialize as Null.
If IsNull(TestVar) Then ' Evaluate variable.
MsgBox "TestVar is Null." ' Indicate if Null.
TestVar = "" ' Make zero-length string.
MsgBox "TestVar is not Null." ' Indicate if not Null.
TestVar = Null ' Make Null.
Loop Until IsNull(TestVar) ' Loop until TestVar is Null.