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Assembly Definitions#

Editor code isn't accessible in a build. Unity has multiple methods to separate and not include this code.

Resolution#

Editor assemblies#

warning

Editor folders don't function under Assembly Definitions

It is recommended to have a Runtime and Editor asmdefs separated into different folders at a root level, unlike the usual paradigm of having editor folders scattered throughout your project.

An editor asmdef should only include Editor under the Platforms selection.
Check that Any Platform is switched off, Deselect All, and then tick Editor only.

Editor Assembly Definition

Preprocessor directives#

Editor code can be wrapped in a UNITY_EDITOR preprocessor #if directive.
This code will only compile when the preprocessor argument is present, which Unity defines when running code from the editor.
The surrounded code will be stripped when building the game.

#if UNITY_EDITOR
using UnityEditor;
#endif

// General Code

#if UNITY_EDITOR
// Other editor-related code
#endif

The result when the preprocessor is stripped must also be valid code to compile.

Replacing UnityEditor code#

If you are finding a need to include editor code in a build then you are abusing the Editor APIs. There is likely a runtime API that will achieve the same outcome, though often requiring completely different methods. Research what you're trying to achieve and implement a solution that does not involve Editor APIs.

For example, SceneAsset is in UnityEditor. Referring to Scenes directly via their asset in runtime code is incorrect. The correct ways to refer to scenes are via their names (a string), their build index (an int), or by loading them with a wrapper like Addressables.
There are also third-party solutions like this one that allow indirect references to scenes via their assets using the inspector.