Conditional compilation symbols

The conditional compilation functionality provided by the #if, #elif, #else, and #endif directives is controlled through pre-processing expressions (§2.5.2) and conditional compilation symbols.

conditional-symbol:
Any identifier-or-keyword except true or false

A conditional compilation symbol has two possible states: defined or undefined. At the beginning of the lexical processing of a source file, a conditional compilation symbol is undefined unless it has been explicitly defined by an external mechanism (such as a command-line compiler option). When a #define directive is processed, the conditional compilation symbol named in that directive becomes defined in that source file. The symbol remains defined until an #undef directive for that same symbol is processed, or until the end of the source file is reached. An implication of this is that #define and #undef directives in one source file have no effect on other source files in the same program.

When referenced in a pre-processing expression, a defined conditional compilation symbol has the boolean value true, and an undefined conditional compilation symbol has the boolean value false. There is no requirement that conditional compilation symbols be explicitly declared before they are referenced in pre-processing expressions. Instead, undeclared symbols are simply undefined and thus have the value false.

The name space for conditional compilation symbols is distinct and separate from all other named entities in a C# program. Conditional compilation symbols can only be referenced in #define and #undef directives and in pre-processing expressions.

Pre-processing expressions

Pre-processing expressions can occur in #if and #elif directives. The operators!, ==,!=, && and || are permitted in pre-processing expressions, and parentheses may be used for grouping.

pp-expression:
whitespace_opt pp-or-expression whitespace_opt

pp-or-expression:
pp-and-expression
pp-or-expression whitespace_opt || whitespace_opt pp-and-expression

pp-and-expression:
pp-equality-expression
pp-and-expression whitespace_opt && whitespace_opt pp-equality-expression

pp-equality-expression:
pp-unary-expression
pp-equality-expression whitespace_opt == whitespace_opt pp-unary-expression
pp-equality-expression whitespace_opt!= whitespace_opt pp-unary-expression

pp-unary-expression:
pp-primary-expression
! whitespace_opt pp-unary-expression

pp-primary-expression:
true
false
conditional-symbol
(whitespace_opt pp-expression whitespace_opt)

When referenced in a pre-processing expression, a defined conditional compilation symbol has the boolean value true, and an undefined conditional compilation symbol has the boolean value false.

Evaluation of a pre-processing expression always yields a boolean value. The rules of evaluation for a pre-processing expression are the same as those for a constant expression (§7.19), except that the only user-defined entities that can be referenced are conditional compilation symbols.

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