Delphi		In the code conditional symbols work like boolean expressions - each of them can be defined or undefined: {$IFDEF SPECIAL} // code here will be compiled if the SPECIAL is defined {$ELSE} // code here will be compiled if the SPECIAL is undefined {$ENDIF} $ELSE is optional, so you can writel something like this: {$IFDEF SPECIAL} // code here will be compiled if the SPECIAL is defined {$ENDIF} You can also use $IFNDEF directive to compile code when some symbol is ubdefined: {$IFNDEF SPECIAL} // code here will be compiled if the SPECIAL is undefined {$ENDIF} The symbol can be defined in the Conditional Defines of project options, or in the code directly: {$DEFINE SPECIAL} {$UNDEF SPECIAL} Conditional symbols can be used for including/excluding some debug code, for creating special versions, etc. If the TRIAL will be defined, the functionality of the following code will be truncated: procedure TMyForm.SaveToFile(FileName: string); begin {$IFDEF TRIAL} ShowMessage('Data cannot be saved in this trial version'); {$ELSE} DataContainer.SaveToFile(FileName); {$ENDIF} end; Delphi predefines several useful symbols, depended from the compiler version (for instance, Delphi 7 defines VER150), from the operation system (MSWINDOWS or LINUX), from the application type (CONSOLE for console applications), etc.
C#		In C# this feature is called preprocessor directives. The following pre-processing directives are available. #define and #undef, which are used to define and undefine, respectively, conditional compilation symbols. #if, #elif, #else, and #endif, which are used to conditionally skip sections of source code. #line, which is used to control line numbers emitted for errors and warnings. #error and #warning, which are used to issue errors and warnings, respectively. #region and #endregion, which are used to explicitly mark sections of source code. #define, #undef, #if, #else and #endif accords to Delphi's $DEFINE, $UNDEF, $IDEF, #ELSE and $ENDIF. C# has new feature, which simplyfies nested if else if else conditions: #elif = else if. #if Enterprise // enterprise options #elif Professional // professional options #else // standard options #endif      New #elif condition.