C# Language Specification

Diagnostic directives

The diagnostic directives are used to explicitly generate error and warning messages that are reported in the same way as other compile-time errors and warnings.

pp-diagnostic:
whitespace_opt # whitespace_opt error pp-message
whitespace_opt # whitespace_opt warning pp-message

pp-message:
new-line
whitespace input-characters_opt new-line

The example:

#warning Code review needed before check-in

#if Debug && Retail

#error A build can't be both debug and retail
#endif

class Test {...}

always produces a warning (“Code review needed before check-in”), and produces a compile-time error (“A build can’t be both debug and retail”) if the conditional symbols Debug and Retail are both defined. Note that a pp-message can contain arbitrary text; specifically, it need not contain well-formed tokens, as shown by the single quote in the word can’t.

6.