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Preprocessor directives are lines included in a program that begin with the character #, which make them different from a typical source code text. They are invoked by the compiler to process some programs before compilation. Preprocessor directives change the text of the source code and the result is a new source code without these directives.
Although preprocessing in C# is conceptually similar to that in C/C++, it is different in two aspects. First, preprocessing in C# does not involve a separate step for preprocessor execution before compilation. It is processed as a part of the lexical analysis phase. Second, it cannot be used to create macros. In addition, the new directives #region and #unregion have been added in C# along with the exclusion of some directives used earlier (#include is a notable directive whose use is replaced with "using" to include assemblies).
Java does not support preprocessor directives.
Techopedia Explains Preprocessor Directive
A preprocessor directive is usually placed in the top of the source code in a separate line beginning with the character "#", followed by directive name and an optional white space before and after it. Because a comment on the same line of declaration of the preprocessor directive has to be used and cannot scroll through the following line, delimited comments cannot be used. A preprocessor directive statement must not end with a semicolon (;). Preprocessor directives can be defined in source code or in the common line as argument during compilation.
Examples for preprocessing directives that can be used in C# include:
- #define and #undef: To define and undefine conditional compilation symbols, respectively. These symbols could be checked during compilation and the required section of source code can be compiled. The scope of a symbol is the file in which it is defined.
- #if, #elif, #else, and #endif: To skip part of source code based on conditions. Conditional sections may be nested with directives forming complete sets.
- #line: To control line numbers generated for errors and warning. This is mostly used by meta-programming tools to generate C# source code from some text input. It is generally used to modify the line numbers and source file names reported by the compiler in its output.
- #error and #warning : To generate errors and warnings, respectively. #error is used to stop compilation, while #warning is used to continue compilation with messages in the console.
- #region and #endregion :To explicitly mark sections of source code. These allow expansion and collapse inside Visual Studio for better readability and reference.
