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.