Conditional compilation directives

The conditional compilation directives are used to conditionally include or exclude portions of a source file. pp-conditional :: pp-if-sectionpp-elif-sections pp-else-section pp-endifpp-if-section :: whitespace#whitespace if whitespacepp-expressionpp-new-lineconditional-section pp-elif-sections :: pp-elif-sectionpp-elif-sectionspp-elif-sectionpp-elif-section :: whitespace#whitespace elif whitespacepp-expressionpp-new-lineconditional-section pp-else-section :: whitespace#whitespace else pp-new-lineconditional-section pp-endif :: whitespace#whitespace endif pp-new-lineconditional-section :: input-sectionskipped-sectionskipped-section :: skipped-section-partskipped-sectionskipped-section-partskipped-section-part :: skipped-characters new-linepp-directiveskipped-characters :: whitespace not-number-signinput-characters not-number-sign :: Any input-character except #

[Note: As indicated by the syntax, conditional compilation directives must be written as sets consisting of, in order, an #if directive, zero or more #elif directives, zero or one #else directive, and an #endif directive. Between the directives are conditional sections of source code. Each section is controlled by the immediately preceding directive. A conditional section may itself contain nested conditional compilation directives provided these directives form complete sets. end note]

A pp-conditional selects at most one of the contained conditional-sections for normal lexical processing: The pp-expressions of the #if and #elif directives are evaluated in order until one yields true. If an expression yields true, the conditional-section of the corresponding directive is selected. If all pp-expressions yield false, and if an #else directive is present, the conditional-section of the #else directive is selected. Otherwise, no conditional-section is selected.
The selected conditional-section, if any, is processed as a normal input-section: the source code contained in the section must adhere to the lexical grammar; tokens are generated from the source code in the section; and pre-processing directives in the section have the prescribed effects.
The remaining conditional-sections, if any, are processed as skipped-sections: except for pre-processing directives, the source code in the section need not adhere to the lexical grammar; no tokens are generated from the source code in the section; and pre-processing directives in the section must be lexically correct but are not otherwise processed. Within a conditional-section that is being processed as a skipped-section, any nested conditional-sections (contained in nested #if...#endif and #region...#endregion constructs) are also processed as skipped-sections.
[Example: The following example illustrates how conditional compilation directives can nest: #define Debug // Debugging on #undef Trace // Tracing off class PurchaseTransaction { void Commit() { #if Debug CheckConsistency(); #if Trace WriteToLog(this.ToString()); #endif #endif CommitHelper(); } }
Except for pre-processing directives, skipped source code is not subject to lexical analysis. For example, the following is valid despite the unterminated comment in the #else section: #define Debug // Debugging on class PurchaseTransaction { void Commit() { #if Debug CheckConsistency(); #else /* Do something else #endif } }
Note, however, that pre-processing directives are required to be lexically correct even in skipped sections of source code.
Pre-processing directives are not processed when they appear inside multi-line input elements. For example, the program: class Hello { static void Main() { System.Console.WriteLine(@"hello, #if Debug world #else Nebraska #endif "); } } results in the output: hello, #if Debug world #else Nebraska #endif
In peculiar cases, the set of pre-processing directives that is processed might depend on the evaluation of the pp-expression. The example: #if X /* #else /* */ class Q { } #endif always produces the same token stream (class Q { }), regardless of whether or not X is defined. If X is defined, the only processed directives are #if and #endif, due to the multi-line comment. If X is undefined, then three directives (#if, #else, #endif) are part of the directive set. end example]