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Conditional Compilation

How can I compile conditionally?

Use {$IFDEF}

You can use $DEFINE to define a symbol to compile your program conditionally.

For example.

{$DEFINE name}

Then, use $IFDEF name to start the conditional compilation. Use $ENDIF some ignored comments to end the conditonal compilation.

 {$IFDEF name}

 // More code ...

 {$ENDIF name}

You can combine multiple {$IFDEF}s too. See the following example (taken from [https://wiki.freepascal.org/\(IF](https://wiki.freepascal.org/\)IF)).

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//Before you need write these conditions to check some conditions:
{$DEFINE SOMETHING}
{$DEFINE SOMETHINGELSE}
{$IFDEF SOMETHING} //Combine $IFDEF to check multiple conditions
  {$IFDEF SOMETHINGELSE}
    {$ModeSwitch advancedrecords}
  {$ENDIF}
{$ENDIF}

Use {$IF}

{$IF} allows you to write complex conditions that cannot be done using {$ifdef aname}.

See the following example from https://wiki.freepascal.org/$IF. You can see two local directives ($DEFINE and $UNDEF) along with and and or logic operators in it.

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{$IF defined(SOMETHING) and defined(SOMETHINGELSE)}//simple and readabl instead of union {$IFDef}`s
  {$ModeSwitch advancedrecords}
{$ENDIF} 

{$IF defined(somthing) or defined(somethingelse)}
  //Whatever you need!
{$ENDIF}

{$IF undefined(what) and defined(somethingelse)}
  //Just for note, Another usage!
{$ENDIF}

References

Useful symbols for conditional compilation

Here is an example contributed by 𝓚𝓸𝓭𝓮𝓩𝔀𝓮𝓻𝓰 🇩🇪 (Unofficial Free Pascal Discord).

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{$IF Defined(DCC) or Defined(VER210) or Defined(VER200) or Defined(VER190) or Defined(VER185) or Defined(VER180) or Defined(VER170) or Defined(VER160) or Defined(VER150) or Defined(VER140) or Defined(VER130) or Defined(VER120) or Defined(VER100) or Defined(VER90) or Defined(VER80)}
  {$DEFINE Delphi} { Delphi }
{$IFEND}

{$IF Defined(DELPHI) and Declared(CompilerVersion) and (CompilerVersion >= 25)}
  {$LEGACYIFEND ON}
{$IFEND}

{$IF Defined(FPC)}
  {$DEFINE Lazarus} { Lazarus and Free Pascal }
{$IFEND}

{$IF Defined(DELPHI) and Declared(CompilerVersion) and (CompilerVersion >= 23)}
  {$DEFINE NameSpace} { Delphis NameSpace feature (eg Winapi.Windows instead of Windows) }
{$IFEND}

{$IF Defined(DELPHI) and Declared(CompilerVersion) and (CompilerVersion >= 20)}
  {$DEFINE UniCode} { Delphis UniCode support }
{$IFEND}

{$IF Defined(WIN32) or Defined(WIN64) or Defined(MSWindows)}
  {$DEFINE Windows} { We are on Windows }
{$IFEND Windows}

{$IF Defined(FPC) and Declared(FPC_VERSION) and (FPC_VERSION >= 3)}
  {$DEFINE UniCode} { FreePascal UniCode support }
{$IFEND}

And here's an example of how you might use these symbols.

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begin
{$IFDEF Delphi}
    // Specific code for Delphi mode
{$ENDIF}

{$IFDEF Lazarus}
    // Specific code for Lazarus and Free Pascal
{$ENDIF}

{$IFDEF Namespace}
    // Specific code for Delphi's namespace
{$ENDIF}

{$IFDEF Windows}
    // Specific code for Windows
{$ENDIF}

{$IFDEF UniCode}
    // Code relies on Unicode
{$ENDIF}
end.