Pre-processing directives

The pre-processing directives provide the ability to conditionally skip sections of source files, to report error and warning conditions, and to delineate distinct regions of source code.

[Note: The term "pre-processing directives" is used only for consistency with the C and C++ programming languages. In C#, there is no separate pre-processing step; pre-processing directives are processed as part of the lexical analysis phase. end note]

pp-directive
:: pp-declarationpp-conditionalpp-linepp-diagnosticpp-region
The following pre-processing directives are available: #define and #undef, which are used to define and undefine, respectively, conditional compilation symbols (9.5.3). #if, #elif, #else, and #endif, which are used to conditionally skip sections of source code (9.5.1). #line, which is used to control line numbers emitted for errors and warnings (9.5.7). #error and #warning, which are used to issue errors and warnings, respectively (9.5.5). #region and #endregion, which are used to explicitly mark sections of source code (9.5.6).
A pre-processing directive always occupies a separate line of source code and always begins with a # character and a pre-processing directive name. White space may occur before the # character and between the # character and the directive name.
A source line containing a #define, #undef, #if, #elif, #else, #endif, or #line directive may end with a single-line comment. Delimited comments (the /* */ style of comments) are not permitted on source lines containing pre-processing directives.
Pre-processing directives are not tokens and are not part of the syntactic grammar of C#. However, pre-processing directives can be used to include or exclude sequences of tokens and can in that way affect the meaning of a C# program. [Example: For example, when compiled, the program #define A #undef B class C { #if A void F() {} #else void G() {} #endif #if B void H() {} #else void I() {} #endif } results in the exact same sequence of tokens as the program class C { void F() {} void I() {} }
Thus, whereas lexically, the two programs are quite different, syntactically, they are identical. end example]