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
class MyClass
{
  int x;
  void Foo()
  {
    # 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 they can be passed to the compiler with the /define:symbol command-line option.

With the #if and #elif directives, you can use the ||, &&, and ! operators to perform or, and, and not operations on multiple symbols. The following directive instructs the compiler to include the code that follows if the TESTMODE symbol is defined and the DEBUG symbol is not defined:

#if TESTMODE && !DEBUG
  ...

Bear in mind, however, that you’re not building an ordinary C# expression, and the symbols upon which you operate have absolutely no connection to variables—static or otherwise.

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.

Here’s a complete list of 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 in compiler output
#error text	text of the error to appear in compiler output
#line [number ["file"] | hidden]	number specifies the line in source code; file is the filename to appear in computer output; hidden instructs debuggers to skip over code from this point until the next #line directive
#region name	Marks the beginning of an outline
#endregion	Ends an outline region
#pragma warning	See below

Pragma Warning

The compiler generates a warning when it spots something in your code that seems unintentional. Unlike errors, warnings don’t ordinarily prevent your application from compiling.

Compiler warnings can be extremely valuable in spotting bugs. Their usefulness, however, is undermined when you get false warnings. In a large application, maintaining a good signal-to-noise ratio is essential if the “real” warnings are to get noticed.

To this effect, the compiler allows you to selectively suppress warnings with the #pragma warning directive. In this example, we instruct the compiler not to warn us about the field Message not being used:

public class Foo
{
  static void Main() { }

  #pragma warning disable 414
  static string Message = "Hello";
  #pragma warning restore 414
}

Omitting the number in the #pragma warning directive disables or restores all warning codes.

If you are thorough in applying this directive, you can compile with the /warnaserror switch—this tells the compiler to treat any residual warnings as errors.