9.5.1 Conditional compilation symbols

9.5.1 Conditional compilation symbols
The conditional compilation functionality provided by the #if, #elif, #else,
and #endif directives is
controlled through pre-processing expressions (§9.5.2) 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.
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.