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MonoBehaviourSingletonBase.cs

using UnityEngine;

/// <summary>
/// provides an easy way to create singletons in unity based on monobehaviours
/// </summary>
/// <typeparam name="T">T must be the same type as the declaring type</typeparam>
public class MBSingleton<T> : MonoBehaviour where T : MonoBehaviour
{
    protected static T _instance;

    public static T Instance
    {
        get
        {
            if (_instance == null)
                GetInstance();

            return _instance;
        }
    }

    /// <summary>
    /// override this in your derived class to decide if the object will be kept between scenes. Default is set to true.
    /// </summary>
    protected virtual bool AddToDontDestroyOnLoad => true;

    private static void GetInstance()
    {
        // check if other objects exist
        var singletons = FindObjectsByType<T>(FindObjectsInactive.Include, FindObjectsSortMode.None);

        if (singletons != null && singletons.Length > 0)
        {
            if (singletons.Length > 1)
                Debug.LogWarning("You should not have more than one singleton monobehaviour present at the same scene. Which singleton is preserved is not guaranteed.", singletons[0]);

            return;
        }

        // otherwise create new singleton
        var go = new GameObject(typeof(T).Name + "_singleton");
        go.AddComponent<T>();
        go.tag = "Singleton";
    }

    /// <summary>
    /// override this function to add custom logic to the construction of the singleton at **runtime**.
    /// </summary>
    /// <remarks>
    /// Construct is not automatically called if the object already exists in the opened scene
    /// </remarks>
    protected virtual void Construct() { }

    /// <summary>
    /// Handles the assignment and construction of the selected singleton. Instead of overriding Awake(), override Construct() instead
    /// </summary>
    protected virtual void Awake()
    {
        if (_instance == null)
        {
            _instance = this as T;

            if (AddToDontDestroyOnLoad)
                DontDestroyOnLoad(gameObject);

            Construct();
        }
        else if (_instance != null && _instance != this)
            Destroy(this);
    }
}