Delphi Basics : $UnDef command

DelphiBasics

$UnDef
Compiler Directive

Undefines a compiler directive symbol - as used by IfDef

{$UnDef Symbol}

Description

The $UnDef compiler directive removes a Symbol definition. By doing so, the symbol is now Off.

When a symbol is on, $IfDef will compile conditional code, and $IfNDef will not.

When a symbol is off, $IfNDef will compile conditional code, and $IfDef will not.

Delphi provides some predefined symbols, such as Console that is on for a console application and off for a GUI application.

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
$IfNDef		Executes code if a conditional symbol has not been defined
$IfOpt		Tests for the state of a Compiler directive

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Example code : Setting up and using a test mode symbol

var
  text : string;

begin
  // Set our code into test mode
  {$Define TESTMODE}

  text := 'We are in test mode';

  // Display the value of text if we are in test mode
  {$IfDef TESTMODE}
  ShowMessage('text = '+text);
  {$EndIf}

  // Switch off test mode
  {$UnDef TESTMODE}

  text := 'We are out of test mode';

  // Display the value of text if we are in test mode
  {$IfDef TESTMODE}
  ShowMessage('text = '+text);
  {$EndIf}
end;

Show full unit code

text = We are in test mode