Conditional Compilation

Unity’s support for the C# language includes the use of directives, which allow you to selectively include or exclude code from compilation, based on whether certain scripting symbols are defined or not defined.

You can read more broadly about how these directives work in C# on the Microsoft C# preprocessor directives page.

Unity has a range of built-in scripting symbols which represent options that you can use in your scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary to selectively include or exclude portions of code from compilation.

For example, the built-in scripting symbol that is set when a player is built for Windows standalone platform is UNITY_STANDALONE_WIN. You can check whether this symbol is defined using a special type of if statement, as follows:

#if UNITY_STANDALONE_WIN

  Debug.Log("Standalone Windows");

#endif

The hash (#) character in front of the if and endif indicates that these statements are “directives”, and are handled during the compilation process, rather than at runtime.

Therefore in the above example, the Debug line is only included for compilation in the Windows standalone build of the project. It is omitted entirely when compiled in the Editor, or in other target builds. This is different to using a regular if .. then .. else structure, which would only bypass the execution of certain portions of code at run time. See further below for full examples.

There are a number of built-in scripting symbols which allow you to selectively compile or omit code based on the selected Platform, the Editor Version, and other miscellaneous system environment scenarios. These built-in scripting symbols are listed below.

In addition, you can define your own scripting symbols either using the Editor UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary, via scripting, or via an asset file, which allows you to control compilation of portions of your code based on arbitrary definitions. See Custom scripting symbols for more information.

Note: Scripting symbols are sometimes referred to as “define symbols”, “preprocessor defines”, or just “defines”.

Platform scripting symbols

Unity automatically defines certain scripting symbols based on the authoring and build target platform. These are as follows:

Define	Function
UNITY_EDITOR	Scripting symbol to call Unity Editor scripts from your game code.
UNITY_EDITOR_WIN	Scripting symbol for Editor code on Windows.
UNITY_EDITOR_OSX	Scripting symbol for Editor code on Mac OS X.
UNITY_EDITOR_LINUX	Scripting symbol for Editor code on Linux.
UNITY_STANDALONE_OSX	Scripting symbol to compile or execute code specifically for Mac OS X (including Universal, PPC and Intel architectures).
UNITY_STANDALONE_WIN	Scripting symbol for compiling/executing code specifically for Windows standalone applications.
UNITY_STANDALONE_LINUX	Scripting symbol for compiling/executing code specifically for Linux standalone applications.
UNITY_STANDALONE	Scripting symbol for compiling/executing code for any standalone platform (Mac OS X, Windows or Linux).
UNITY_WII	Scripting symbol for compiling/executing code for the Wii console.
UNITY_IOS	Scripting symbol for compiling/executing code for the iOS platform.
UNITY_IPHONE	Deprecated. Use UNITY_IOS instead.
UNITY_ANDROID	Scripting symbol for the Android platform.
UNITY_LUMIN	Scripting symbol for the Magic Leap OS platform. You can also use PLATFORM_LUMIN. Note that the Lumin platform is no longer supported.
UNITY_TIZEN	Scripting symbol for the Tizen platform.
UNITY_TVOS	Scripting symbol for the Apple TV platform.
UNITY_WSA	Scripting symbol for Universal Windows PlatformAn IAP feature that supports Microsoft’s In App Purchase simulator, which allows you to test IAP purchase flows on devices before publishing your application. More info See in Glossary. Additionally, NETFX_CORE is defined when compiling C# files against .NET Core and using .NET scripting backendA framework that powers scripting in Unity. Unity supports three different scripting backends depending on target platform: Mono, .NET and IL2CPP. Universal Windows Platform, however, supports only two: .NET and IL2CPP. More info See in Glossary.
UNITY_WSA_10_0	Scripting symbol for Universal Windows Platform. Additionally WINDOWS_UWP is defined when compiling C# files against .NET Core.
UNITY_WEBGL	Scripting symbol for WebGLA JavaScript API that renders 2D and 3D graphics in a web browser. The Unity WebGL build option allows Unity to publish content as JavaScript programs which use HTML5 technologies and the WebGL rendering API to run Unity content in a web browser. More info See in Glossary.
UNITY_FACEBOOK	Scripting symbol for the Facebook platform (WebGL or Windows standalone).
UNITY_ANALYTICS	Scripting symbol for calling Unity AnalyticsAbbreviation of Unity Analytics See in Glossary methods from your game code. Version 5.2 and above.
UNITY_ASSERTIONS	Scripting symbol for assertions control process.
UNITY_64	Scripting symbol for 64-bit platforms.

Editor version Scripting symbols

Unity automatically defines certain scripting symbols based on the version of the Editor that you are currently using.

Given a version number X.Y.Z (for example, 2019.4.14), Unity exposes three global scripting symbols in the following formats: UNITY_X, UNITY_X_Y and UNITY_X_Y_Z.

Here is an example of scripting symbols exposed in Unity 2019.4.14:

Define	Function
UNITY_2019	Scripting symbol for the release version of Unity 2019, exposed in every 2019.Y.Z release.
UNITY_2019_4	Scripting symbol for the major version of Unity 2019.4, exposed in every 2019.4.Z release.
UNITY_2019_4_14	Scripting symbol for the minor version of Unity 2019.4.14.

You can also compile code selectively based on the earliest version of Unity required to compile or execute a given portion of code. Given the same version format as above (X.Y), Unity exposes one global #define in the format UNITY_X_Y_OR_NEWER, that you can use for this purpose.

Other scripting symbols

The other scripting symbols Unity defines are:

Define	Function
CSHARP_7_3_OR_NEWER	Defined when building scripts with support for C# 7.3 or newer.
ENABLE_MONO	Scripting backend #define for Mono.
ENABLE_IL2CPP	Scripting backend #define for IL2CPPA Unity-developed scripting back-end which you can use as an alternative to Mono when building projects for some platforms. More info See in Glossary.
ENABLE_VR	Defined when the target build platform supports VRVirtual Reality More info See in Glossary. Does not imply that VR is currently enabled or that the necessary plug-insA set of code created outside of Unity that creates functionality in Unity. There are two kinds of plug-ins you can use in Unity: Managed plug-ins (managed .NET assemblies created with tools like Visual Studio) and Native plug-ins (platform-specific native code libraries). More info See in Glossary and packages needed to support VR are installed.
NET_2_0	Defined when building scripts against .NET 2.0 API compatibility level on Mono and IL2CPP.
NET_2_0_SUBSET	Defined when building scripts against .NET 2.0 Subset API compatibility level on Mono and IL2CPP.
NET_LEGACY	Defined when building scripts against .NET 2.0 or .NET 2.0 Subset API compatibility level on Mono and IL2CPP.
NET_4_6	Defined when building scripts against .NET 4.x API compatibility level on Mono and IL2CPP.
NET_STANDARD_2_0	Defined when building scripts against .NET Standard 2.0 API compatibility level on Mono and IL2CPP.
NET_STANDARD_2_1	Defined when building scripts against .NET Standard 2.1 API compatibility level on Mono and IL2CPP.
NET_STANDARD	Defined when building scripts against .NET Standard 2.1 API compatibility level on Mono and IL2CPP.
NETSTANDARD2_1	Defined when building scripts against .NET Standard 2.1 API compatibility level on Mono and IL2CPP.
NETSTANDARD	Defined when building scripts against .NET Standard 2.1 API compatibility level on Mono and IL2CPP.
ENABLE_WINMD_SUPPORT	Defined when Windows Runtime support is enabled on IL2CPP. See Windows Runtime Support for more details.
ENABLE_INPUT_SYSTEM	Defined when the Input System package is enabled in Player SettingsSettings that let you set various player-specific options for the final game built by Unity. More info See in Glossary.
ENABLE_LEGACY_INPUT_MANAGER	Defined when the legacy Input ManagerSettings where you can define all the different input axes, buttons and controls for your project. More info See in Glossary is enabled in Player Settings.
UNITY_SERVER	Defined when the Server Build setting is enabled in Build Settings
DEVELOPMENT_BUILD	Defined when your script is running in a player which was built with the “Development BuildA development build includes debug symbols and enables the Profiler. More info See in Glossary” option enabled.

Testing precompiled code

Below is an example of how to use the precompiled code. It prints a message that depends on the platform you have selected for your target build.

First of all, select the platform you want to test your code against by going to File > Build Settings. This displays the Build Settings window; select your target platform from here.

Select the platform you want to test your precompiled code against and click Switch Platform to tell Unity which platform you are targeting.

Create a script and copy/paste the following code:

using UnityEngine;
using System.Collections;

public class PlatformDefines : MonoBehaviour {
  void Start () {

    #if UNITY_EDITOR
      Debug.Log("Unity Editor");
    #endif

    #if UNITY_IOS
      Debug.Log("iOS");
    #endif

    #if UNITY_STANDALONE_OSX
        Debug.Log("Standalone OSX");
    #endif

    #if UNITY_STANDALONE_WIN
      Debug.Log("Standalone Windows");
    #endif

  }          
}

To test the code, click Play Mode. Confirm that the code works by checking for the relevant message in the Unity console, depending on which platform you selected - for example, if you choose iOS, the message “Unity iOS” is set to appear in the console.

In C# you can use a CONDITIONAL attribute which is a more clean, less error-prone way of stripping out functions. See ConditionalAttribute Class for more information. Note that common Unity callbacks (ex. Start(), Update(), LateUpdate(), FixedUpdate(), Awake()) are not affected by this attribute because they are called directly from the engine and, for performance reasons, it does not take them into account.

In addition to the basic #if compiler directive, you can also use a multiway test in C#:

#if UNITY_EDITOR

    Debug.Log("Unity Editor");

#elif UNITY_IOS

    Debug.Log("Unity iOS");

#else

    Debug.Log("Any other platform");

#endif