In Unity3D having a singleton class is very useful, whether for "global" state or simply for the convenience of having a static accessor so you don't have to have lots of: FindObjectOfType(typeof(Builder)) as Builder;

So you code up a C# singleton and then realize that you actually need it to be a MonoBehavior, not just a ScriptableObject -- say you want the singleton to run coroutines, or have a transform, or any other MonoBehavior feature. But monobehaviors can't be initialized with a constructor.

So what you want is a monobehavior pseudo singleton pattern. The unity community wiki has some good info about the various options but all require boilerplate.

So I wrote a parametrized generic pseudo singleton monobehavior class. I attempted to compile all the best advice out there, so it is threadsafe, has some extra stuff for setting a parent, doesn't implement an Awake, and initializes lazily.

The key was the C# where keyword, used to constrain the parameterizing type. So in this case both the class is a MonoBehavior and its parameterization type is a MonoBehavior.

So now this blog has its first code section:

using UnityEngine;
using System.Collections;

/// 

/// MONOBEHAVIOR PSEUDO SINGLETON ABSTRACT CLASS
/// usage    : best is to be attached to a gameobject but if not that is ok,
///         : this will create one on first access
/// example    : '''public sealed class MyClass : Singleton {'''
/// references    : http://tinyurl.com/d498g8c
///         : http://tinyurl.com/cc73a9h
///         : http://unifycommunity.com/wiki/index.php?title=Singleton
/// 
public abstract class Singleton : MonoBehaviour where T : MonoBehaviour
{

    private static T _instance = null;

    /// 

    /// gets the instance of this Singleton
    /// use this for all instance calls:
    /// MyClass.Instance.MyMethod();
    /// or make your public methods static
    /// and have them use Instance
    /// 
    public static T Instance {
        get {
            if (_instance == null) {
                _instance = (T)FindObjectOfType (typeof(T));
                if (_instance == null) {

                    string goName = typeof(T).ToString ();          

                    GameObject go = GameObject.Find (goName);
                    if (go == null) {
                        go = new GameObject ();
                        go.name = goName;
                    }

                    _instance = go.AddComponent ();                   
                }
            }
            return _instance;
        }
    }

    /// 

    /// for garbage collection
    /// 
    public virtual void OnApplicationQuit ()
    {
        // release reference on exit
        _instance = null;
    }

    // in your child class you can implement Awake()
    // and add any initialization code you want such as
    // DontDestroyOnLoad(go);
    // if you want this to persist across loads
    // or if you want to set a parent object with SetParent()

    /// 

    /// parent this to another gameobject by string
    /// call from Awake if you so desire
    /// 
    protected void SetParent (string parentGOName)
    {
        if (parentGOName != null) {
            GameObject parentGO = GameObject.Find (parentGOName);
            if (parentGO == null) {
                parentGO = new GameObject ();
                parentGO.name = parentGOName;
            }
            this.transform.parent = parentGO.transform;
        }
    }
}

Full gist is here.