Jon Jagger
jon@jaggersoft.com
Table of Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Notes DownloadECMA-334 C# Language Specificationpreviousnextprevious at this levelnext at this level 9 Lexical structureprevious at this levelnext at this level 9.5 Pre-processing directivesprevious at this levelnext at this level 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: Paragraph 31 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. Paragraph 41 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. 2 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]
{ JSL }
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