Boolean operators

An expression can be prefixed with a boolean operator, which causes the expression to evaluate to a boolean value.

Operator	Description
!	inverts the boolean value; converts 0 or null to true
!!	inverts the boolean value twice; converts 0 or null to false

In the following example, the controls are disabled or hidden when the bound list is empty:

<!-- disabled=true when size=0 -->
<MyAddressControl disabled="${!user::addresses.size}"/>

<!-- visible=false when size=0 -->
<MyAddressControl visible="${!!user::addresses.size}"/>

Applicability

A boolean operator is applicable to any expression type, not just boolean expressions. However, restrictions apply depending on which expression type is used.

Markup extension	Applicable
fx:Evaluate	all non-content expressions
fx:Observe	all non-content expressions
fx:Synchronize	only if the binding source implements WritableValue<Boolean>