Specifically, like this in C#:
#if (DEBUG)
Debug Stuff
#endif
C# has the following preprocessor directives:
#if
#else
#elif // Else If
#endif
#define
#undef // Undefine
#warning // Causes the preprocessor to fire warning
#error // Causes the preprocessor to fire a fatal error
#line // Lets the preprocessor know where this source line came from
#region // Codefolding
#endregion