Directivas compilación {$IFDEF...}..{$ELSE}..

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Código Delphi [-]
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;

Código Delphi [-]
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;