How to create an Editor class for a generic class

Question

For all my MonoBehaviour scripts, instead of them inheriting from MonoBehaviour they inherit from a generic class like so:

public abstract class MonoBase<T> : MonoBehaviour where T : MonoBase<T> { }

public class Example : MonoBase<Example> { }

Now, I would like to make an editor script for all those MonoBase classes, and the closest I have gotten to a solution that will compile is :

[CustomEditor(typeof(MonoBase<>))]
public class MonoBaseEditor<T> : Editor where T : MonoBase<T>
{
    public override void OnInspectorGUI() {
        this.DrawDefaultInspector();

        MonoBase<T> myScript = (MonoBase<T>)this.target;
        if (GUILayout.Button("Build Object")) {
        }
    }
}

But of course it doesn't really work.

Am I missing something or is there really no way to make a generic editor class like it's mentioned in an answer here.

Quoting your link, "The Editor class is derived from ScriptableObject and as such it can't be a generic type. You need a concrete class and the class name has to match the filename. You could have a generic base class like this but the actual editors must not contain a generic parameter." That looks pretty definitive to me. It matches the tests I've done and it sounds like you're observing the same results. Is there a reason to believe this is not the answer? — DMGregory
– DMGregory ♦, Commented Jan 31, 2020 at 13:50
Well the answer is from 4 years ago, and I though about posting here in case someone has a different solution to my problem or if it became possible in some way in the meantime :D That answer states that you can't make a generic editor class (which I don't really need), I just need an editor script that will work with my generic class. — Milos Romanic
– Milos Romanic, Commented Jan 31, 2020 at 13:56

troien · Accepted Answer · 2020-01-31 15:51:09Z

Well, you can make an Editor class for it, but Unity can't instantiate a generic one, when you instantiate a generic class, you must specify the generic type, as example, when you try do do this C# will throw an error: List<> l = new List<>();. And there is no way for Unity to safely assume or guess what T should be.

You can get it to work by creating a custom editor for each T, so that Unity can instantiate the custom editor where T is known, see the other answer of Ed Marthy on how to do this

We can also make a regular Editor, we just can't cast target to MonoBase<T> because we don't know T, And if you did know for sure that T is Example in this case, you should just write a custom editor for Example instead of MonoBase<> in the first place...

The closest thing you can cast 'target' to is the (Non-generic) type that MonoBase<> inherrits (MonoBehaviour in your example).

[CustomEditor(typeof(MonoBase<>), true)] // <-- the 'true' is important, it makes sure that classes that inherrit MonoBase use this editor aswell
public class ExampleEditor : Editor
{
    public override void OnInspectorGUI()
    {

        if (GUILayout.Button("Build Object"))
        {
            MonoBehaviour possible = (MonoBehaviour)target;
            Debug.Log(possible.name);
        }
    }
}

In your case, because the T in MonoBase<T> has to be a MonoBase<T> itself. You really can't cast to anything MonoBase. If the where clause of T would be something like where T : MonoBehaviour instead of where T : MonoBase<T>, and you would be willing to use an interface rather then abstract class, then you could make it so you can cast target to IMonoBase<MonoBehaviour>. Using the answer to this question.

public interface IMonoBase<out T> where T : MonoBehaviour
{
    IMonoBase<T> ExampleMethod();
}

public abstract class MonoBase<T> : MonoBehaviour, IMonoBase<T> where T : MonoBehaviour
{
    public abstract IMonoBase<T> ExampleMethod();
}

public class Example : MonoBase<Example>
{
    public override IMonoBase<Example> ExampleMethod()
    {
        return this;
    }
}

Editor script:

[CustomEditor(typeof(MonoBase<>), true)]
public class ExampleEditor : Editor
{
    public override void OnInspectorGUI()
    {
        if (GUILayout.Button("Build Object"))
        {
            IMonoBase<MonoBehaviour> test = (IMonoBase<MonoBehaviour>)target;
            IMonoBase<MonoBehaviour> example = test.ExampleMethod();
            Debug.Log(example is IMonoBase<Example>);
        }
    }
}

I realized I could of just said T was MonoBehaviour instead of MonoBase<T> but was so interested to find out other ways of making this work. Both solutions are great, thanks <3 — Milos Romanic
– Milos Romanic, Commented Jan 31, 2020 at 15:03

Ed Marty · Accepted Answer · 2020-01-31 15:02:06Z

Another option I’ve used is to have separate editor classes for each class, but they all extend from the base editor class and do nothing themselves.

public class MonoBaseEditor<T> : Editor where T : MonoBase<T>
{
    public override void OnInspectorGUI() {
        this.DrawDefaultInspector();

        MonoBase<T> myScript = (MonoBase<T>)this.target;
        if (GUILayout.Button("Build Object")) {
        }
    }
}

[CustomEditor(typeof(Example))]
public class ExampleEditor : MonoBaseEditor<Example>
{}

[CustomEditor(typeof(AnotherClass))]
public class AnotherClassEditor : MonoBaseEditor<AnotherClass>
{}

[CustomEditor(typeof(ThirdClass))]
public class ThirdClassEditor : MonoBaseEditor<ThirdClass>
{}

Alturis · Accepted Answer · 2023-05-20 13:33:07Z

This actually works more simply now with more recent versions of Unity. You don't need to worry about the filename matching and you just need an interface for OnInspectorGUI()

e.g.

// ScriptableIndex<T> : General purpose ScriptableObject automatic indexer
public class ScriptableIndex<T> : ScriptableObject, IScriptableIndex where T : ScriptableObject

public interface IScriptableIndex
{
    public void OnInspectorGUI();
}

[CustomEditor(typeof(ScriptableIndex<>), true)]
public class EditorScriptableIndex : Editor
{
    public override void OnInspectorGUI()
    {
        IScriptableIndex editObject = target as IScriptableIndex;
        if ( editObject != null )
            editObject.OnInspectorGUI();
    }
}

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