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IsNull - function of language VBScript

Returns a boolean value that specifies whether an expression contains no valid data.
In the PROMOTIC system it is better to use the Pm.IsValid method.
Boolean IsNull(Variant expression)
expression(Variant) Any valid expression.
Return value:
true - If expression is Null, i.e. it contains no valid data.
false - Otherwise
If expression contains of more than one variable, then 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 Val = Null or If Val <> Null, are always false. This is because any expression containing a Null is itself Null, and therefore, false.
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Dim Val, bCheck
bCheck = IsNull(Val)   ' Returns false
Val = Null
bCheck = IsNull(Val)   ' Returns true
Val = Empty
bCheck = IsNull(Val)   ' Returns false
PROMOTIC 9.0.28 SCADA system documentation MICROSYS, spol. s r.o.

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