C# Language Pocket Reference

Preprocessor Directives

Preprocessor directives supply the compiler with additional information about regions of code. The most common preprocessor directives are the conditional directives, which provide a way to include or exclude regions of code from compilation. For example:

#define DEBUG
using System;
class MyClass {
  static int x = 5;
  static void Main( ) {
  # if DEBUG
    Console.WriteLine("Testing: x = {0}", x);
  # endif
  }
}

In this class, the statement in Foo is compiled as conditionally dependent upon the presence of the DEBUG symbol. If we remove the DEBUG symbol, the statement is not compiled. Preprocessor symbols can be defined within a source file (as we have done) and can be passed to the compiler with the /define: symbol command-line option.

The #error and #warning symbols prevent accidental misuse of conditional directives by making the compiler generate a warning or error when given an undesirable set of compilation symbols. See Table 1-5 for a list of preprocessor directives and their actions.

Table 1-5. Preprocessor directives

Preprocessor directive	Action
#define symbol	Defines symbol
#undef symbol	Undefines symbol
#if symbol [ operator symbol2 ] ...	symbol to test; operators are ==, !=, &&, and ||, followed by #else, #elif, and #endif
#else	Executes code to subsequent #endif
#elif symbol [ operator symbol2 ]	Combines #else branch and #if test
#endif	Ends conditional directives
#warning text	text of the warning to appear ...