The VBScript programming language does not short-circuit evaluation of Boolean conditions. VBScript evaluates all the terms in a Boolean logical expression, even if the expression can be established as True or False without evaluating all its terms. For example, in the following example, both <
statement1> and <
statement2> are evaluated, even if <
statement1> resolves to False:
<statement 1> AND <statement 2>
To avoid errors, check that all values and objects are not Null before attempting to use them.
The following examples:
- demonstrate incorrect and correct usage of logical expressions
- take into consideration how logical expressions are evaluated
value.Name is evaluated even when its value is Null. This causes an error.
Sub namecheck(value) If Not IsNull(value) And value.Name = "aName" Then ' ... End If End Sub
The code is correct on the condition that
value is an object that contains the
Name property. The code runs without errors.
Sub namecheck(value) If Not IsNull(value) And Not IsEmpty(value) Then If value.Name = "aName" Then ' ... End If End If End Sub