ECMA-334 C# Language Specification

9.5: Pre-processing directives

The pre-processing directives provide the ability to conditionally skip sections of source files, to report error and warning conditions, and to delineate distinct regions of source code. [Note: The term "pre-processing directives" is used only for consistency with the C and C++ programming languages. In C#, there is no separate pre-processing step; pre-processing directives are processed as part of the lexical analysis phase. end note]

pp-directive
pp-declaration
pp-conditional
pp-line
pp-diagnostic
pp-region

The following pre-processing directives are available:

A pre-processing directive always occupies a separate line of source code and always begins with a # character and a pre-processing directive name. White space may occur before the # character and between the # character and the directive name.

A source line containing a #define, #undef, #if, #elif, #else, #endif, or #line directive may end with a single-line comment. Delimited comments (the /* */ style of comments) are not permitted on source lines containing pre-processing directives.

Pre-processing directives are not tokens and are not part of the syntactic grammar of C#. However, pre-processing directives can be used to include or exclude sequences of tokens and can in that way affect the meaning of a C# program. [Example: For example, when compiled, the program 
#define A  
#undef B  
class C  
{  
   #if A  
   void F() {}  
   #else  
   void G() {}  
   #endif  
   #if B  
   void H() {}  
   #else  
   void I() {}  
   #endif  
}
results in the exact same sequence of tokens as the program 
class C  
{  
   void F() {}  
   void I() {}  
}

Thus, whereas lexically, the two programs are quite different, syntactically, they are identical. end example]

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