2.18 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 {
  int x;
  public void Foo(  ) {
  # if DEBUG
  # warning "Debug mode is ON"
    Console.WriteLine("Testing: x = {0}", x);
  # endif
  }
}

In this class, the statement in Foo is compiled conditionally, dependent upon the presence of the user-selected DEBUG symbol. If you remove the DEBUG symbol, the statement isn't compiled. Preprocessor symbols can be defined within a source file as just shown 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 given an undesirable set of compilation symbols.

2.18.1 Preprocessor Directives

The C# language supports the preprocessor directives shown in Table 2-4.

Table 2-4. Preprocessor directives

Preprocessor directive	Action
#define symbol	Defines symbol
#undef symbol	Undefines symbol
#if symbol [operator symbol2] ...	symbol to test; operator: ==, !=, &&, || followed by #else, #elif, #endif
#else	Executes code to subsequent #endif
#elif symbol [operator symbol2]	Combines #else branch and #if test
#endif	Ends conditional directives
#warning text	text: warning text to appear in compiler output
#error text	text: error message to appear in compiler output
#line number [file]	number specifies line in source code; file is the filename to appear in computer output
#region name	Marks beginning of outline
#end region	Ends an outline region