This section discusses the C# language's preprocessor directives:
While the compiler does not have a separate preprocessor, the directives described in this section are processed as if there was one; these directives are used to aid in conditional compilation. Unlike C and C++ directives, you cannot use these directives to create macros.
A preprocessor directive must be the only instruction on a line.
For more information, see 2.5 Pre-processing directives.
See Also
C# Programmer's Reference | Conditional Methods Tutorial