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 results in the exact same sequence of tokens as the program Thus, whereas lexically, the two programs are quite different, syntactically, they are identical. end example]