
9 Lexical structure
9.5 Pre-processing directives
9.5.4 Conditional compilation directives
Paragraph 11 The conditional compilation directives are used to conditionally include or exclude portions of a source file. pp-conditional :: pp-if-section pp-elif-sectionsopt pp-else-sectionopt pp-endif pp-if-section :: whitespaceopt # whitespaceopt if whitespace pp-expression pp-new-line conditional-sectionopt pp-elif-sections :: pp-elif-section pp-elif-sections pp-elif-section pp-elif-section :: whitespaceopt # whitespaceopt elif whitespace pp-expression pp-new-line conditional-sectionopt pp-else-section :: whitespaceopt # whitespaceopt else pp-new-line conditional-sectionopt pp-endif :: whitespaceopt # whitespaceopt endif pp-new-line conditional-section :: input-section skipped-section skipped-section :: skipped-section-part skipped-section skipped-section-part skipped-section-part :: skipped-charactersopt new-line pp-directive skipped-characters :: whitespaceopt not-number-sign input-charactersopt 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]
Paragraph 21 A pp-conditional selects at most one of the contained conditional-sections for normal lexical processing:
#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 #endifIn 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]
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