Unity: Assign instance of abstract object in the inspector

Question

In my game, I have a magic system that uses a command-pattern to store the steps. How can I assign an abstract command not derived from MonoBehavior or ScriptableObject in the inspector (or just not hard-coding it)?

Below I have the minimum example, note that SpellCommand does not derive from MonoBehavior nor ScriptableObject.

public abstract class SpellCommand {
    public SpellCommand (int power = -1) {
        this.power = power;
    }
    public abstract void Cast();
}

And here is what a spell might look like

public class SampleSpell : SpellCommand {
    public SampleSpell(int power) : base(power: power) { }

    public override void Cast() {
        Debug.Log("Sample Spell Cast");
    }
}

And finally a MonoBehavior to invoke it

public class TestSpellInvoker : MonoBehaviour
{
    public SpellCommand testSpell;

    void Start() {
        testSpell.Cast();
    }
}

Since I know Unity doesn't support serialization of abstract classes not derived from MonoBehavior, I wrote my own inspector script to assign and display the contents of testSpell but whenever I enter play mode, it seems to be nulled out.

I've considered removing all polymorphism and simply using a large Enum to represent subclasses, but that would introduce a ton of complexity and it feels very much against the spirit of C# and OOP. I've also considered using ScriptableObjects, but that would require (in my case) having potentially thousands of ScriptableObject instances saved as files that I would then have to wrangle (although that being said my knowledge of ScriptableObjects is pretty elementary so I might be missing something).

Answer 1

1. Your abstract class and implementors should be Serializable. If it's not, your data won't be stored either in scene, or in a prefab.
2. I haven't used it in a MonoBehaviour but this works in a ScriptableObject. Use SerializableReference instead of SerializableField. As long as you follow the rules in the documentation (Serializable, not from UnityEngine.Object, not a value type, not a dictionary), then you can store the implementors instances.

NOTE: If you do this, you'll need to adjust your custom inspector to display the data depending on the implementor.