VBScript in a Nutshell, 2nd Edition

Name

IsNull Function

Syntax

IsNull(expression)

expression

Use: Required

Data Type: Any

An expression containing string or numeric data.

Return Value

Boolean (True or False).

Description

Determines whether expression contains is Null.

Rules at a Glance

• If the expression passed to IsNull is Null, True is returned; otherwise, IsNull returns False.

• All variables in expression are checked for null values. If a null value is found in any one part of the expression, True is returned for the entire expression.

• In VBScript, Null is a separate data type that can take on a single value, Null. It is used to indicate that data is missing. Because it represents missing data, all expressions that include a Null value also result in a Null value. This makes perfect sense. For instance, if we have an array containing two valid months of sales data and a Null representing the third month’s sales data, the quarter’s sales data should also be Null, since accurate data for the quarter is not available.

Programming Tips and Gotchas

• IsNull is useful when returning data from a database. You should check field values in columns that allow Nulls against IsNull before assigning the value to a collection or other variable. This stops the common “Invalid Use of Null” error from occurring.

• IsNull is the only way to evaluate an expression containing a null. For example, the seemingly correct statement:

  If varMyVar = Null Then

  always evaluates to False, even if varMyVar is null. This occurs because the value of an expression containing ...