SingletonSimple.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

// by @kurtdekker - to make a simple Unity singleton that has no
// predefined data associated with it, eg, a high score manager.
//
// To use: access with SingletonSimple.Instance
//
// To set up:
//	- Copy this file (duplicate it)
//	- rename class SingletonSimple to your own classname
//	- rename CS file too
//
// DO NOT PUT THIS IN ANY SCENE; this code auto-instantiates itself once.
//
// I do not recommend subclassing unless you really know what you're doing.

public class SingletonSimple : MonoBehaviour
{
	// This is really the only blurb of code you need to implement a Unity singleton
	private static SingletonSimple _Instance;
	public static SingletonSimple Instance
	{
		get
		{
			if (!_Instance)
			{
				_Instance = new GameObject().AddComponent<SingletonSimple>();
				// name it for easy recognition
				_Instance.name = _Instance.GetType().ToString();
				// mark root as DontDestroyOnLoad();
				DontDestroyOnLoad(_Instance.gameObject);
			}
			return _Instance;
		}
	}

	// implement your Awake, Start, Update, or other methods here...
}

As I just recently realized, a MonoBehaviour will not be destroyed as long as there is a reference to it even if the underlying gameobject is destroyed. So, for this use case, wouldn't it make sense to instantly destroy the associated game object instead of making it survive scene changes with DontDestroyOnLoad? After all you can always get your hands on the Instance to destroy it if that would be needed.