ECMA-334 C# Language Specification

9.5.3: Declaration directives

The declaration directives are used to define or undefine conditional compilation symbols.

pp-declaration
whitespaceopt # whitespaceopt define whitespace conditional-symbol pp-new-line
whitespaceopt # whitespaceopt undef whitespace conditional-symbol pp-new-line
pp-new-line
whitespaceopt single-line-commentopt new-line

The processing of a #define directive causes the given conditional compilation symbol to become defined, starting with the source line that follows the directive. Likewise, the processing of an #undef directive causes the given conditional compilation symbol to become undefined, starting with the source line that follows the directive.

Any #define and #undef directives in a source file must occur before the first token (9.4) in the source file; otherwise a compile-time error occurs. In intuitive terms, #define and #undef directives must precede any "real code" in the source file.

[Example: The example: 
#define Enterprise  
#if Professional || Enterprise  
#define Advanced  
#endif  
namespace Megacorp.Data  
{  
   #if Advanced  
   class PivotTable {...}  
   #endif  
}
is valid because the #define directives precede the first token (the namespace keyword) in the source file. end example]

[Example: The following example results in a compile-time error because a #define follows real code: 
#define A  
namespace N  
{  
   #define B  
   #if B  
   class Class1 {}  
   #endif  
}
end example]

A #define may define a conditional compilation symbol that is already defined, without there being any intervening #undef for that symbol. [Example: The example below defines a conditional compilation symbol A and then defines it again. 
#define A  
#define A

For compilers that allow conditional compilation symbols to be defined as compilation options, an alternative way for such redefinition to occur is to define the symbol as a compiler option as well as in the source. end example]

A #undef may "undefine" a conditional compilation symbol that is not defined. [Example: The example below defines a conditional compilation symbol A and then undefines it twice; although the second #undef has no effect, it is still valid. 
#define A  
#undef A  
#undef A
end example]