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IsNull - function of language VBScript

Description:
Returns a boolean value that indicates whether an expression contains no valid data.

In the PROMOTIC system it is better to use the method: Pm.IsValid.

Syntax:
IsNull(expression As Variant) As Boolean
Calling:
b = IsNull(expression)
Parameters:
expression(Variant) Any valid expression.
Return Values:
true - If expression is Null, i.e. it contains no valid data.
false - otherwise
Note:

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 empty string (""), which is sometimes referred to as a null string.

Caution !!! Use the IsNull function to determine whether an expression contains a Null value. Expressions that you might expect to evaluate to true under some circumstances, such as If Var = Null or If Var <> Null, are always false. This is because any expression containing a Null is itself Null, and therefore, false.

Example:
Dim Var, bCheck
bCheck = IsNull(Var) ' Returns false
Var = Null
bCheck = IsNull(Var) ' Returns true
Var = Empty
bCheck = IsNull(Var) ' Returns false
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