C# Preprocessor Directives (C#) This section contains information about the following C# preprocessor directives. #if #else #elif #endif #define #undef #warning #error #line #region #endregion #pragma #pragma warning #pragma checksum See the individual topics for more information and examples. Although the compiler does not have a separate preprocessor, the directives described in this section are processed as if there were one. A preprocessor directive must be the only instruction on a line. The C# Language. C# Programming Guide. C# Programmer's Reference (C#) MSDN Code Gallery - Home. Language Specification at C# Online.NET (CSharp-Online.NET) Visual C# Developer Center.