C ＃ entry series (29) -- preprocessing commands

Concept

   The source code of the program is specified , Preprocessing instructions instruct the compiler how to process the source code .C# The preprocessing instructions in are processed by the compiler , Preprocessing instructions are processed as part of the lexical analysis phase , There is no separate preprocessing phase .

   Preprocessor instructions are never translated into commands in executable code , But it affects all aspects of the compilation process . The preprocessing instruction provides conditional skipping of sections in the source file 、 Report errors and warning conditions , And the ability to depict different areas of source code .

   When compiling the basic version of the software , Using preprocessor instructions prevents the compiler from compiling code related to additional functions . When writing code that provides debugging information , You can also use preprocessor instructions .

Common preprocessing instructions

Rule of grammar

Some important syntax rules to pay attention to when using preprocessing instructions ：

• Preprocessing instructions always occupy a single line of source code ;

• The preprocessing instruction does not need to use a semicolon as the end flag ;

• Preprocessing instructions must be in # The character starts with the preprocessing instruction name , stay # Before characters and # There can be spaces between characters and instructions

• contain #define、#undef、#if、#elif、#else、#endif or #line The source code line of the instruction can end with a single line comment .

• The preprocessing instruction is neither a tag , Neither C# Components of syntactic grammar . however , You can include or exclude tag sequences with preprocessing instructions , And can affect... In this way C# Meaning of procedure .

Preprocessing expressions

Preprocessing expressions can appear in #if and #elif In the instruction . In preprocessing expressions,... Is allowed !、==、!=、&& and || Operator , And you can use parentheses to group .

When referenced in a preprocessing expression , The defined conditional compilation symbol has a Boolean value true, Undefined conditional compilation symbols have Boolean values false.