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. For more information on how these directives work in C#, refer to Microsoft's documentation on C# preprocessor directives.

Unity has a range of predefined symbols you can use in your scripts to selectively include or exclude sections of code from compilation. For example, the symbol that's defined in project builds for Windows standalone platform is . You can check whether this symbol is defined using a special type of statement:

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

The hash () character in front of and indicates that these statements are directives that are handled during the compilation process, rather than at runtime. In the previous example, the Debug line is only included for compilation in the Windows standalone build of the project. When compiled in the Unity Editor or in other target builds, it's omitted entirely. This is different to using a regular if statements, which might only bypass the execution of certain sections of code at runtime.

You can use the and directives to check multiple conditions:

#if UNITY_EDITOR    Debug.Log("Unity Editor");#elif UNITY_IOS    Debug.Log("Unity iOS");#else    Debug.Log("Any other platform");#endif

There are several predefined symbols which allow you to selectively compile or omit code based on the selected Platform, the Editor Version, and other system environment scenarios. For the full list of Unity's predefined symbols, refer to Unity scripting symbol reference.

You can also define your own scripting symbols using the Editor, via scripting, or via an asset file. For more information, refer to Custom scripting symbols.

Preprocessor directives are not always the most appropriate or reliable way to conditionally include or exclude code. Alternative approaches are listed here.

You can use the C# attribute which is a cleaner, less error-prone way of stripping out functions. For more information, refer to ConditionalAttribute Class. Common Unity callbacks such as , , , , aren't affected by this attribute because they're called directly from the engine and, for performance reasons, it doesn't take them into account.

The recommended approach for conditional compilation at a high level is to organize your scripts in assemblies with associated Assembly Definition files. If the code you want to conditionally include or exclude is in an assembly, you can configure Define Constraints on the assembly definition to, for example, only compile the code when a given version of a package is present in the project.

Instead of conditional compilation, you can enforce conditional execution of the compiled code using standard statements. For example, Unity's scripting symbol is an unreliable test for 64-bit architecture, so it's preferable to do the following instead:

if (IntPtr.Size == 4){    // 32 bit code}else{    // 64-bit code}