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.