Chapter 24. C preprocessor directives

Chapter 24. C# preprocessor directives

If you have ever read C/C++ codes before, you can't have missed those strange looking #define and #include statements found at the beginning of the source codes. These special keywords, which begin with a # sign, are called preprocessor directives.

Preprocessor directives, though part of the source codes, are not compiled into IL codes. They are only used by the C# compiler for making certain decisions during compilation. This will become clear as you go through the examples which follow.

Table 24.1 shows the preprocessor directives supported in C#: ^[1]

^[1] C/C++ developers: there have been significant changes to the C/C++ preprocessor directives in C#. Notice that #include is no longer a valid directive and #ifdef has been replaced with #if . #define cannot be used to define constants or write macro functions. #pragma has been also removed.

Table 24.1. Valid preprocessor directives in C#

Preprocessor directive	Usage syntax	Comments
#define	#define <symbol>	Used to define and undefine a symbol
#undef	#undef <symbol>
#if	#if <symbol> [operator <symbol>] ...	Used for conditional compilation
#else	#else
#elif	#elif <symbol> [operator <symbol>] ...
#endif	#endif
#warning	#warning text	Used to show a warning or error message during compilation
#error	#error text
#region	#region name	Used to mark and unmark a region of code
#endregion	endregion
#line [1]	line [number [ " file_name " ] default]	Used to modify the compiler's line number and file name output for errors and warnings

^[1] The #line preprocessor directive is seldom used, and will not be discussed in this book.