The Null Value

Purpose

The Null keyword is used with a:

Variant to indicate that it intentionally contains no valid data.
Handle data type to indicate a null handle.
API function to pass a null value for ByRef parameters.

Syntax

Variant | Handle = Null
Boolean = IsNull(Variant | Handle)

Description

Variant variables are not set to Null unless you explicitly assign Null to them, so if you don't use Null in your application, you don't have to write code that tests for and handles it. You can assign Null as follows:

Dim v As Variant = Null

You can use the variant function IsNull to test if a Variant variable contains Null:

If IsNull(variant) Then Print "Variant contains Null"

Data "#Null#" can be used with Data lines to initialize a Variant.

A handle data type can simply be compared with Null. Here Null is defined as CHandle(0).

If hWnd == Null Then Print "Handle is Null"

For API functions that have parameters declared as ByRef, the Null value may be passed (if that API function can handle a Null value), in contrast to the number 0.

Example

OpenW 1

Local a As Variant, b As Handle, x%

Print IsNull(b) // result True

a = ""

Print IsNull(a) // result 0

b = 2

Print IsNull(b) // result False

b = 0

Print IsNull(b) // result True

x% = Null

Print IsNull(x%) // result False

Print "Press any key to close"

KeyGet x%

CloseW 1

Remarks

Null is commonly used in database applications to indicate unknown or missing data. Because of the way it is used in databases, Null has some unique characteristics:

- Expressions involving Null always result in Null. Thus, Null is said to "propagate" through expressions; if any part of the expression evaluates to Null, the entire expression evaluates to Null.

- Passing Null, a Variant containing Null, or an expression that evaluates to Null as an argument to most functions causes the function to return Null.

- Null values propagate through intrinsic functions that return Variant data types.