C# Language Specification

Declaration directives

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

pp-declaration:
whitespace_opt # whitespace_opt define whitespace conditional-symbol pp-new-line
whitespace_opt # whitespace_opt undef whitespace conditional-symbol pp-new-line

pp-new-line:
whitespace_opt single-line-comment_opt 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 (§2.4) 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.

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.

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
}

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

#define A
#define A

A #undef may “undefine” a conditional compilation symbol that is not defined. 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

4.