Inspired by this post.

The SerializeReference attribute, added in Unity 2019.3, makes it possible to serialize references to interfaces and abstract classes.

The SubclassSelector attribute allows you to easily set subclasses of those abstract classes in the Editor that are serialized by SerializeReference attribute.

• Easily set subclass by popup.
• [New] Type finding by fuzzy finder.
• [New] Override the type name and path by the AddTypeMenu attribute.
• [New] Available CustomPropertyDrawer for subclasses.
• [New] Restore values of previous object from JSON when subclass is changed. (required Unity 2021.3 or later)
• [New] Copy & Paste the subclass properties. (required Unity 2021.3 or later)
• [New] Clear & reset the subclass properties. (required Unity 2021.3 or later)

See below for the reason for the limitation of versions less than Unity 2021.3.

https://blog.unity.com/engine-platform/serializereference-improvements-in-unity-2021-lts

Download any version from releases.

Releases: https://github.com/mackysoft/Unity-SerializeReferenceExtensions/releases

Or, you can add this package by opening PackageManager and entering

https://github.com/mackysoft/Unity-SerializeReferenceExtensions.git?path=Assets/MackySoft/MackySoft.SerializeReferenceExtensions

from the Add package from git URL option.

If you are specifying a version, enter #{VERSION} at the end, as shown below.

https://github.com/mackysoft/Unity-SerializeReferenceExtensions.git?path=Assets/MackySoft/MackySoft.SerializeReferenceExtensions#1.1.9

Or, you can install this package from the Open UPM registry.

More details here.

openupm add com.mackysoft.serializereference-extensions

using System;
using UnityEngine;

public class Example : MonoBehaviour {

	// The type that implements ICommand will be displayed in the popup.
	[SerializeReference, SubclassSelector]
	ICommand m_Command;

	// Collection support
	[SerializeReference, SubclassSelector]
	ICommand[] m_Commands = Array.Empty<ICommand>();

	void Start () {
		m_Command?.Execute();

		foreach (ICommand command in m_Commands) {
			command?.Execute();
		}
	}

	// Nested type support
	[Serializable]
	public class NestedCommand : ICommand {
		public void Execute () {
			Debug.Log("Execute NestedCommand");
		}
	}

}

public interface ICommand {
	void Execute ();
}

[Serializable]
public class DebugCommand : ICommand {

	[SerializeField]
	string m_Message;

	public void Execute () {
		Debug.Log(m_Message);
	}
}

[Serializable]
public class InstantiateCommand : ICommand {

	[SerializeField]
	GameObject m_Prefab;

	public void Execute () {
		UnityEngine.Object.Instantiate(m_Prefab,Vector3.zero,Quaternion.identity);
	}
}

// Menu override support
[AddTypeMenu("Example/Add Type Menu Command")]
[Serializable]
public class AddTypeMenuCommand : ICommand {
	public void Execute () {
		Debug.Log("Execute AddTypeMenuCommand");
	}
}

[Serializable]
public struct StructCommand : ICommand {
	public void Execute () {
		Debug.Log("Execute StructCommand");
	}
}

The SubclassSelector attribute supports types that meet the following conditions.

• Public
• Not abstract
• Not generic
• Not unity object
• Serializable attribute is applied.

It is a limitation of SerializeReference of Unity.

When serializing a SerializeReference reference, the type name, namespace, and assembly name are used, so if any of these are changed, the reference cannot be resolved during deserialization.

To solve this problem, UnityEngine.Scripting.APIUpdating.MovedFromAttribute can be used.

Also, this thread will be helpful.

• https://forum.unity.com/threads/serializereference-data-loss-when-class-name-is-changed.736874/
• https://issuetracker.unity3d.com/issues/serializereference-serialized-reference-data-lost-when-the-class-name-is-refactored

I welcome feature requests and bug reports in issues and pull requests.

If you feel that my works are worthwhile, I would greatly appreciate it if you could sponsor me.

GitHub Sponsors: https://github.com/sponsors/mackysoft