C# Language Specification

Conditional compilation directives

The conditional compilation directives are used to conditionally include or exclude portions of a source file.

pp-conditional:
pp-if-section pp-elif-sections_opt pp-else-section_opt pp-endif

pp-if-section:
whitespace_opt # whitespace_opt if whitespace pp-expression pp-new-line conditional-section_opt

pp-elif-sections:
pp-elif-section
pp-elif-sections pp-elif-section

pp-elif-section:
whitespace_opt # whitespace_opt elif whitespace pp-expression pp-new-line conditional-section_opt

pp-else-section:
whitespace_opt # whitespace_opt else pp-new-line conditional-section_opt

pp-endif:
whitespace_opt # whitespace_opt endif pp-new-line

conditional-section:
input-section
skipped-section

skipped-section:
skipped-section-part
skipped-section skipped-section-part

skipped-section-part:
skipped-characters_opt new-line
pp-directive

skipped-characters:
whitespace_opt not-number-sign input-characters_opt

not-number-sign:
Any input-character except #

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.

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.

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.

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