#Boolean Functions

XPath has just four boolean functions. Booleans work differently than in C# in one key way: XPath has "effective boolean value" rules that automatically convert other types to boolean in conditional contexts.

#boolean()

Converts a value to boolean using XPath's effective boolean value rules.

Signature: boolean($value as item()*) as xs:boolean

boolean("hello")    => true     (: non-empty string :)
boolean("")         => false    (: empty string :)
boolean(42)         => true     (: non-zero number :)
boolean(0)          => false    (: zero :)
boolean(())         => false    (: empty sequence :)
boolean((1, 2))     => true     (: non-empty sequence starting with a node or value :)

Effective boolean value rules: | Input | Result | |-------|--------| | Empty sequence () | false | | Sequence starting with a node | true | | Single xs:boolean | Its value | | Single xs:string | true if non-empty | | Single number | true if non-zero and not NaN |

C# equivalent: There's no single C# equivalent — C# doesn't implicitly convert strings or numbers to booleans.

Why this matters: In XPath, you can write if (//error) then ... and it works — the sequence of error nodes is automatically converted to true if any exist. You don't need if (count(//error) > 0).

#true()

Returns true. Needed because XPath doesn't have boolean literals.

Signature: true() as xs:boolean

true()   => true

Note: Unlike C#'s true, this is a function call with parentheses. XPath 4.0 also allows the keyword true without parentheses in some contexts.

#false()

Returns false.

Signature: false() as xs:boolean

false()   => false

#not()

Logical negation.

Signature: not($value as item()*) as xs:boolean

not(true())               => false
not(false())              => true
not(())                   => true    (: empty sequence is "falsy" :)
not(//error)              => true if no error elements exist
not(contains("abc", "x")) => true

C# equivalent: !value

Common pattern:

//book[not(@out-of-print)]          (: books without the out-of-print attribute :)
//item[not(price > 100)]            (: items not over $100 :)