[Unity3D] Generic singleton class initialization for MonoBehaviours

MonoSingleton.cs

using UnityEngine;
using System;
using UnityEngine.SceneManagement;



/// <summary>
/// This class handles Singleton instance and managing. Just inherit from this and voyalla!
/// <para></para>
/// </summary>
/// <typeparam name="T"></typeparam>
public class MonoSingleton<T> : MonoBehaviour where T : MonoSingleton<T>
{
    static Func<T> valueFactory = () =>
    {
        UnityEngine.Object[] oj = FindObjectsOfType(typeof(T), true);
        T scriptType;

        if (oj.Length == 0)
        {
            GameObject instant = new GameObject(typeof(T).ToString() + "_Static");
            DontDestroyOnLoad(instant);
            instant.AddComponent(typeof(T));
                
            scriptType = instant.GetComponent<T>();
        }
        else
            scriptType = ((MonoBehaviour)oj[0]).GetComponent<T>();

        SceneManager.sceneLoaded += (scriptType as MonoSingleton<T>).CheckCopyCats;

        return scriptType;
    };

    private static Lazy<T> lazyInstance =
            new Lazy<T>(valueFactory);
    public static T Instance
    {
        get
        {
            if (lazyInstance.Value == null)
                lazyInstance = new Lazy<T>(valueFactory);
            return lazyInstance.Value;
        }
    }

    protected virtual void CheckCopyCats(Scene sc, LoadSceneMode mode)
    {
            
        UnityEngine.Object[] oj = FindObjectsOfType(typeof(T), true);
        if (oj.Length <= 1)
            return;

        for (int i = oj.Length-1; i>0; i--)
        {
            if (Instance != ((MonoBehaviour)oj[i]).GetComponent<T>())
                Destroy(((MonoBehaviour)oj[i]).gameObject);
        }
    }
}

For Non-MonoBehaviour singletons please check this >> https://gist.github.com/brenocogu/8c06d7d710630bbbcd5a79bb932ae663

[14/12/2022] Gist marked as "[PREVIEW]" due to unstable initialization with multiple singleton instance (due to the "Don't destroy on load"). This error only happens with a pre-set value for a MonoSingleton instance (i.e. a GameObject) within a scene that can be returned to at any give point in the application run-time (e.g: A MonoSingleton existent on the "Menu" Scene can be duplicated if the user returns to the menu)

[20/12/2022] Gist fixed. Now MonoSingleton handles multiple instances of the same singleton. The script can now be used for both pre-set game-objects or instancing game-objects. Profiling with some stress test provided enough info to sustain that the CopyCat check method isn't a big performance killer.