The Not expression syntax

TALES not expressions convert a value to boolean and then perform a logical NOT on that value. In other words the not expression is true if the expression it acts upon is false, and vice-versa.

The syntax of a not expression is another TALES expression; typically a path expression but it doesn't need to be:

not:other_expression

Not expressions are included in the main ZptSharp package

Support for not expressions is included in the main ZptSharp NuGet package. Additionally, not expressions are one of the standard expression types activated by AddStandardZptExpressions().

Not expressions have their own rules for boolean conversion

The TALES not expression does not use the same rules as TAL does for boolean coercion. Not expressions use their own logic for converting a value to boolean. Only a very specific set of values are treated as false and for some value-types (based upon struct) it is impossible to provide a value which corresponds to false (System.DateTime comes to mind).

Examples

This expression will evaluate to false if the path expression here/anObject resolves to an object created with new object(). This is because the path expression is treated as true by the rules of the not expression, and so the overall expression is false.

not:here/anObject

This expression will evaluate to true. This is because the string expression evaluates to an empty string. Because System.String implements IEnumerable, and an empty string is an enumerable of zero-length, the not expression treats it as false. This means that the overall expression is true.

not:string: