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 scripts 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, 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:

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:

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:

Testing precompiled code

下面是如何使用预编译代码的示例。该示例根据为目标构建选择的平台打印一条消息。

首先，通过 File > Build Settings 选择要测试代码的平台。随后将显示 Build Settings 窗口；从此处选择目标平台。

选择要测试预编译代码的平台，然后单击 Switch Platform 向 Unity 告知您所需的目标平台。

创建脚本并复制/粘贴以下代码：

using UnityEngine;
using System.Collections;

public class PlatformDefines : MonoBehaviour {
  void Start () {

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

    #if UNITY_IOS
      Debug.Log("Unity 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 messages in the Unity console, depending on which platform you selected - for example, if you choose iOS, the messages “Unity Editor” and “Unity iOS” are set to appear in the console.

在 C# 中，可使用 CONDITIONAL 属性，这是一种更简洁、更不容易出错的函数剥离方式。请参阅 ConditionalAttribute 类以了解更多信息。 请注意，常见的 Unity 回调（例如：Start()、Update()、LateUpdate()、FixedUpdate()、Awake()）不受此属性的影响，因为它们是直接从引擎调用的，并且出于性能原因，此属性不会考虑它们。

除了基本的 #if 编译器指令外，还可在 C# 中使用多路测试：

#if UNITY_EDITOR

    Debug.Log("Unity Editor");

#elif UNITY_IOS

    Debug.Log("Unity iOS");

#else

    Debug.Log("Any other platform");

#endif