Delphi Basics : $IfNDef command

DelphiBasics

$IfNDef
Compiler Directive

Executes code if a conditional symbol has not been defined

{$IfNDef Symbol}
... Code to run when the symbol is defined ...
{$Else}
... Code to run when the symbol is not defined ...
{$EndIf}

Description

The $IfNDef compiler directive starts a section of conditionally compiled code. Only if the specified Symbol has not been defined.

Symbols come in two types. Predefined in Delphi, such as Console when running a console application. And defined by the $Define compiler direcctive.

$IfDef is very useful when developing code, allowing various sections to be compiled when testing.

Related commands

$Define		Defines a compiler directive symbol - as used by IfDef
$Else		Starts the alternate section of an IfDef or IfNDef
$EndIf		Terminates conditional code compilation
$IfDef		Executes code if a conditional symbol has been defined
$IfOpt		Tests for the state of a Compiler directive
$UnDef		Undefines a compiler directive symbol - as used by IfDef

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Example code : Setting up and using a user defined symbol

begin
  // Set our code into dangerous mode
  {$Define DANGERMODE}

  // Are we out of danger?
  {$IfNDef DANGERMODE}
  ShowMessage('We are out danger at present');
  {$Else}
  ShowMessage('We are in danger mode!');
  {$EndIf}

  // Switch off danger mode
  {$UnDef DANGERMODE}
end;

Show full unit code

We are in danger mode!